A player tried to use the spell enlarge/reduce on his PC while in the mouth of the Tarrasque to shrink himself, then go deeper into its airway and to try to collapse its lung by enlarging himself inside of it. I imagine there's no real rule for this but I felt silly just denying it, because it seemed like a cool idea that you could only probably do with a creature as large as the Tarrasque.

Can you crawl into a gargantuan creature’s mouth or into its airway via reduce/enlarge and attack its brain or internal organs?

Rules support for a Rule of Cool attempt

You are in the territory of a GM ruling, and since called shots and attacks on specific body parts aren't covered in the rules, this will probably break down into a three step process (at least) if your innovative PC wants to make this work. It won't be easy, but why not let them try?

1. 
   

   Ability Check: Nature or Arcana

   
   
   

   The player needs to do the research to find out the weak points of a Tarrasque, and if it in fact has lungs. Let's say say that they pass that ability check, and they find out that Tarrasque does indeed have lungs¹. (DM ruling; nothing says yes or no in the monster description). So let's roll with this, or you just stop here if the ruling is "no lungs."

   
   


2. 
   

   Ability Check, Opposed Athletics

   
   
   

   Now the PC wants to get into the throat, and then expand into Huge size (see below) without getting swallowed by the Tarrasque.

   
   
   
   • A non-trivial technical limitation here is that a Tarrasque can swallow a Large creature, so your PC needs to be able to turn into a Huge Creature, like a T Rex or a Stone Giant. This may call for True Polymorph, rather than Enlarge or Reduce; or, perhaps Polymorph into a Huge creature and be able to "reduce" (per the spell) to become Large to get in, and the dispel / stop being reduced to turn Huge. Unless your PC is already Large, getting from Large to Huge will take some work. Wild Shape? Shapechange? True Polymorph? Note that the PC can't stack enlarge spells to make this work.
   
   

   
   

   The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap. (Basic Rules, p. 81; Combining Magical Effects).

   
   

   
   
   

   Assign this research project to your PC: most players enjoy a challenge. Research Topic: How to get Huge inside a Tarrasque's Throat?
   
   
   
   Once inside the Tarrasque's head/throat, the difficulty is in getting to the "just right" spot in order to expand and try to collapse some lungs, or just choke the beast, since the Tarrasque has this particular ability:

   
   
   

   
   

   Swallow. The Tarrasque makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Tarrasque, and it takes 56 (16d6) acid damage at the start of each of the Tarrasque's turns.

   
   

   
   
   

   Since the character isn't yet grappled by the bite, per the bite attack description (MM, Tarrasque) the issue of being swallowed is in doubt. As DM, this is a fine time to call for an opposed Athletics check (see rules on Contests) with the character's Athletics ability versus the Tarrasque's(Strength) check. The Tarrasque's +10 modifier ups the challenge here. There is a chance to succeed, or a chance to fail. Feel the tension building! If the PC fails there is a chance for bite, swallow, and more as consequences of leaping into the Tarrasque's maw.

   
   
   

   Consider whether or not Advantage or Disadvantage applies here. The DM can always rule that circumstantial advantage, or disadvantage, applies. (Basic Rules, p. 4)

   
   
   

   Granted, you as DM could assign a high DC (25-30, maybe higher) for this very difficult task without creating an opposed check. (DMG, DC's; PHB; DC's).

   
   
   

   If the PC manages to pull this off, there is good reason that they are able to remain stuck in the Tarrasque since the thing trapped inside of it is larger than a Large creature; it won't be barfed up by doing damage to it internally per the "barf 'em up" feature of the Tarrasque:

   
   
   

   
   

   If the Tarrasque takes 60 damage or more on a single turn from a creature inside it, the Tarrasque must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the Tarrasque. If the Tarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

   
   

   
   
   

   OK, the PC has collapsed the lungs, or cut off the airway, now what? The PC is likely restrained, and may wish to remain so for a while. Why?

   
   

It will probably take a while for the Tarrasque to die.

Suffocating

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds). When a creature runs out of breath, it can survive for a number of rounds equal to its Constitution modifier (minimum 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can’t regain hit points or be stabilized until it can breathe again. Basic Rules, p. 65)

The Tarrasque has a Constitution of 30; +10 ability modifier. It can hold its breath for 11 minutes, and then when it is out of breath it can choke to death in 10 rounds. (Another minute). You, the DM, need to decide whether or not the PC also has to deal with a similar problem while stuck there in the Tarrasque's throat?

Is oxygen starvation on the table for our brave adventurer? There are some magic items and spells, and some class abilities, that might mitigate this ... Necklace of Adaptation for example.

The DM can also adapt the suffocation rule a bit, as needed, to help with making this play out more smoothly.

If this whole thing succeeds, the PC can wear his, or her, "I Made the Tarrasque Gag!" t-shirt with pride.

This isn't a case of an "I Win" button; this is more like a mini-adventure within the larger "fight the Tarrasque" adventure, and it isn't a sure thing. That's a good thing: what reward if there's no risk?

¹ As @MarkTO noted in this comment:
_{Many creatures do not have a combined airway/esophagus. This is how
they can drink endlessly without having to pause to breathe like
humans. Therefore, climbing into its mouth and enlarging might make it
gag but wouldn't cut off its airway. That makes this a part of the aforementioned research project. See this Zoology stack exchange question for details (warning, graphic imagery).}

You could...but you probably shouldn't. And it may not really matter anyway.

There are a couple of things to consider when determining the possibility of this tactic. The first is whether or not you can get inside the oral cavity of a Tarrasque. This thing is huge, so using Reduce in order to do so seems functionally possible.

The problem is that there really isn't any sort of mechanics for going inside creatures. The bigger issue is likely in entering the Tarrasque in the first place - especially if you go in through the mouth.

The Tarrasque has both a Bite and Swallow attack. If you're inside it's mouth, it's very likely going to use that against you. THe other is the secondary part of the Swallow attack that concerns me more about going inside this dreaded monstrosity:

While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 56 (16d6) acid damage at the start of each of the tarrasque's turns.

Being inside, or at least in the stomach, seems like a particularly dangerous place.

Called Shots

In addition, and more importantly, 5e doesn't have a called shot mechanic. Attacks are much more generalized. What you may consider is reducing the AC, but even that doesn't happen to the Swallowed creature, so I'm not sure there's a precedent for that and would suggest keeping the AC the same.

Getting up there

You should also consider how they're getting into that mouth. This thing is 50' tall, they're going to need to fly up there somehow.

Rule of Cool

However, this is a pretty cool idea. As a DM, you need to balance cool ideas against powerful creatures - and the Tarrasque is one of the most powerful. If you're saying this is a viable tactic, allow called shots, etc. - then you're significantly reducing it's CR as well.

The problem is that once inside the mouth, they are still a candidate for the Tarrasque's Bite and Swallow attacks. I'd also have to heavily consider that if they're already in the mouth/throat of the Tarrasque, the Swallow is likely just going to happen. They don't need to bite them first to get them in their mouth.

It's likely not the best idea functionally even if it sounds cool at first pass.

There's not much point.

Let's be frank – rules as written there is nothing in 5e about climbing into heads. But setting that aside, let's imagine that you reduce your size enough to climb into a tarrasque's ear or nostril. Well, that's pretty dangerous, now it can attack you even more efficiently. But let's set that aside as well.

Let's say you get into it's ear. Well ears are in no way connected to brain, so not much advantage gained there. You can attempt to hack at it, but that is still the same attack as you could have been doing before you got reduced, only now you substract 1d4 (or more) as per the Reduce effect. So not very effective.

The same story goes for other orifices. Your DM might be kind enough to give you some advantage for attacking some soft tissues if you get to those, but by that time he might also count you as effectively swallowed. The trouble with anything vital is that it's usually inside the creature and to get to it, you would need to slice it open ...which is what you were trying to do in the fist place.

Bonus.

This is going to be completeley in the DM fiat territory, but the rules for reduce state that any item you drop while reduced returns to its original size immediately. So consider bringing a stack of cartoonishly large spears, swords or pikes when getting into that ear and then dropping them carelessly.

The other answers are great and cover most things, but there are a few things missed:

1. The PC is likely to take damage as well. In addition to all the ways others mentioned, also by the very act that is damaging the tarrasque: the PC is being crushed.




2. Ok, so let's say you collapsed a lung and it is now suffocating. How many lungs does it have? If it has 2, like humanoid default, then 1 lung might be filling and not contributing, but the other one(s) might still be operating at least at partial, possibly full, capacity.




3. (2b, really) So you enlarge in the wider tube going down into the lungs instead of down in the main part of the lungs to try and ruin the main air entry. Now you probably take even more crushing damage to your PC than you did before.

Since it will likely be very difficult to climb back out, especially now that you have caused more slippery bodily fluids to enter the area you are at. You may be stuck, crushed, possibly also suffocating. This is probably a suicide mission to be attempted by someone who is willing to sacrifice themselves to save the day.

1. If the T hit with bite, your buddy is grappled and restrained.


2. Enlarge/Reduce doesn’t not state anything about freedom of movement due to size change, still grapple and restrained.


3. Enlarge/Reduce, now has disadvantage on Strength checks


4. Enlarge/Reduce is a concentration spell for up to 10 rounds.


5. Grappled/Restrained you have 0 speed.


6. You have 33% chance navigating to lung verse nose or stomach.


7. No active light, face obscured light conditions.


8. T can use Chomp as Legendary action to Swallow


9. Squeeze into smaller places rules for internal organs.

There are several mechanics that can help navigate through this, but is up to DM to decide. No one likes a straight up no, but with such an epic creature, I wouldn’t let a RoC defeat such a creature.