We are playing a homebrew campaign based on the 5e core rulebooks, only allowing monster races (Monsters of the Multiverse). One of my players chose to play a bugbear rune knight.
The bugbear states: "Sneaky. You are proficient in the Stealth skill. In addition, without squeezing, you can move through and stop in a space large enough for a small creature."
Playing a rune knight he is able to use Giant's Might, "If you are smaller than Large, you become Large, along with anything your are wearing. If you lack the room to become Large, your size doesn't change."
Considering magic-users in the party the Enlarge/Reduce spell might come into play somewhere down the line, boosting the bugbear to be Huge in size.
My question is: Is a bugbear – regardless of its size – able to fit into spaces small creatures fit into, without squeezing?
Best Answer
The squeezing rules have problems
The Sneaky trait gives you this ability:
The space required for a small creature is 5x5 feet, just as for a medium creature (see the table Size Categories, p. 191 PHB). That means, unless your Medium bugbear is made large by a spell, the ability does exactly nothing for them, as the rules are written.
According to the normal rules for squeezing (p. 192 PHB):
Since the space of both Small and Medium creatures is 5x5 feet, and Small is one size smaller than Medium, this means that Medium creatures cannot usefully squeeze into a Small creature's space, and cannot squeeze into a Tiny creature's smaller space at all.
The text of Sneaky as written would mean that no matter how large the character is at the time, they will be able to move through a space of 5x5 without squeezing. When they are only Large, this would actually be what one would expect -- they can squeeze into the next smaller space box.
If they are Huge, they normally cannot enter a Medium creature size space in combat, not even with squeezing. I think while Sneaky takes away the need to squeeze, it still should not grant them the ability to enter such a space (but it is not written that way).
The intent behind all of this is likely that you can squeeze into a Tiny creature's space if you are Medium (i.e. into 2.5 x 2.5 feet), or into a Medium creature's space if you are Large, or a Large creature's space if you are Huge.1
So, what to do with all of this? You as the DM will have to rule on this, whether you want to stick to the rather nonsensical rule as written (they can move into a 5x5 space without penalty, no matter how large), or whether you want to use another interpretation that makes more sense to you, like allowing the creature to squeeze into the next smaller space size.
In either case, the Bugbear character would be able to move into that space without squeezing and without incurring all the disadvantages associated with squeezing.
P.S. Squeezing and its penalties are a rule that only makes sense in combat, when speed and advantages to attacks matter. Outside of combat, you can move through much smaller spaces than the space you need to control to fight effectively. This speaks against interpreting "large enough" as referring to the space the creature's body takes up, instead of the space it needs to operate effectively in combat.
1 In older editions, Small characters often had advantages in getting through narrow openings that Medium characters could not, to offset some of the disadvantages of being small. It is possible that it is intentional that Small characters are able to squeeze into Tiny spaces, while Medium characters cannot, but it still is problematic, as it deprives Medium characters of any useful way to squeeze.