A sneaky thief wants to infiltrate a building through its chimney. Can she do that? PHB p. 193 "Creature size" says a creature "squeezes" when going through a small space:
A creature can squeeze through a space that is large enough for a creature one size smaller than it.
Presumably, that means a creature can go through a space small enough for it to squeeze in, but not smaller. But how small this space can be? "A space that is large enough for a creature one size smaller" is unclear. How large is "large enough"? The same chapter says
A creature's space is <...> not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example.
So if not 5 feet wide, how small that space can be? The answer to Can Medium creatures squeeze into smaller spaces? partially answers this by saying
While the small creature would be able to do it easily, the medium creature would need to squeeze.
But this is basically redefining what "squeezing" means for Medium creature through what it does for Small ones.
In other words, what are the minimal space a (let's say Medium) creature can go through? Since trivial doors are usually not 5 feet wide, I'm pretty sure it is smaller than 5 feet, but how smaller? If answer depends on the fact if we're playing on grid, I'd like to hear both options.