What minimal space can a creature go through when squeezing?

Question

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.

Answer 1

Ask the GM, the rules are not precise and exacting

There are no exacting dimensions on the biggest medium creature or the smallest small creature and whether the first could squeeze into a space the size of the latter. You'll have to ask the GM. That said, there are rules:

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If you are not using a grid

Then a creature can squeeze into a space that it otherwise could not fit into but that a creature one size smaller could fit into without squeezing:

[...] A creature can squeeze through a space that is large enough for a creature one size smaller than it. [...]

What this means in terms of exact dimensions is never stated and so is up to the GM.

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If you are using a grid

We know a little more about creatures and size as the section on Creature Size states:

Each creature takes up a different amount of space. The Size Categories table shows how much space a creature of a particular size controls in combat. [...]

Of course, this isn't their physical size, it's what they control during combat. That said, there are clear definitions of this right afterwards.

A tiny creature controls a 2.5*2.5 space
A small or medium creature controls a 5*5 space
A large creature controls a 10*10 space A huge creature controls a 15*15 space
A gargantuan creature controls at least a 20*20 space

This doesn't tell us how much physical space creatures of a given size category take up, so any real limit on squeezing has to be made by the GM, but it is some sort of start.

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Personally, I abstract squeezing away almost entirely

If my party consists of medium creatures, I am simply going to design a dungeon with sections that either do not require squeezing, do require squeezing, or cannot be traversed even while squeezing. If there are creatures of various sizes, I'll just decide which paths each size of creature can fit into. The exact dimensions don't matter there.

The place I do use slightly more exact dimensions is when I simply apply the rules on control by letting a small creature squeeze into a 2.5*2.5 space and a large creature squeeze into a 5*5 space (such as to avoid an area of effect). Using this, a medium creature is unique in that squeezing mid-combat does not make it now control less space.

Answer 2

The squeezing rules are not about passing through narrow openings

The squeezing rules are about movement in combat, nothing more. They are specifically in the Combat section of the rules.

To adjudicate a creature passing through a small opening or tunnel, the DM has to make a ruling: doable, impossible or doable with an ability check.