Can a Rune Knight fighter use the Giant's Might feature to become Large in a 5-foot-wide passage (by "squeezing" as soon as they become Large)?

Question

Rune Knight fighters can grow in size using the Giant's Might feature, whose description states (Tasha's Cauldron of Everything, p. 45; emphasis mine):

If you are smaller than Large, you become Large, along with anything you are wearing. If you lack the room to become Large, your size doesn’t change.

I know that Large creatures can typically squeeze into 5-foot-wide corridors, but does that mean you can become Large in a smaller space and immediately be squeezed? Or does the "lack of room" imply that you shouldn't be squeezed?

The rule on squeezing into a smaller space says (PHB, p. 192):

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space.

So it seems possible for a Rune Knight to become Large, then squeeze into a narrow corridor - but could you start in the narrow corridor and immediately squeeze? Assume for now this is used during combat, where squeezing rules apply.

Answer 1

This is largely up to the DM to determine, but it's probably worth talking about the reason for that clause.

In general, sections like these are just meant to tell everyone involved what the options are: "you become large" or "you don't grow bigger". It implicitly tells the DM and player that you can't use this power to grow large and break open a coffin you're trapped in, or burst your chains, or explode a purple worm from inside, or whatever.

Both players and DMs have a long history of attempting to treat any size-changing effect as if size change were an unstoppable force, whether it's a creature doing it or an item ("I put one end of the Compliant Staff against the wall and the other end against the stone door, and command it to grow!") Worse, how to rule the power can easily become a point of contention: "I use my Giant's Might to become big and explode out of the coffin!" "Okay you become large. You are crushed to jelly against the inside of the coffin, roll up a new character." "WHAT?! THAT IS NOT HOW IT WORKS!"

As a result, size change effects generally have some kind of explanation of what happens when there's not enough space, which is usually "it fails" or "you get as big as you can given the space".

In other words, there isn't really a balance issue whether the DM decides you become large and have to squeeze or the ability just doesn't work (though I'd probably have you squeeze because we all saw Alice in Wonderland get stuck inside the White Rabbit's house and it's hilarious). Rather, the benefit is in giving a clear and concise answer about what happens when squeezing isn't a possibility.

Answer 2

As far as I can find there is no official ruling. As you have stated all the applicable rules, and the RAW are still ambiguous (no clear RAI), I would say with a fair amount of confidence that this is a individual DM rule decision on the interpretation of the phrase:

If you lack the room to become Large

Mechanically there is no end difference to becoming large outside the tunnel/small space and then entering it by squeezing, compared to just becoming large in the space. Based on this I personally see no issue allowing enlargening into a squeeze.