Large creatures movement penalties based on space available where movement ends

Question

I know creatures can squeeze so if I am a large creature I can squeeze my way through a 5-foot square hall spending 1 extra foot for every foot it moves there. But, can a large creature ends its movement on a less than 10 foot square such as 1, 2, or 3 5-foot squares together (cases A, B, C below)? If it can then it has disadvantage on attack rolls and Dexterity saving throws and attack rolls against the creature have advantage while it’s in the smaller space, right?

So what if the large creature is amorphous (like oozes)? It ignores the extra foot cost for squeezing for sure according to RAW, but what about the other penalties if it ends its movement on a smaller square? I just wonder if oozes can end their movement on 5, 5x2 and 5x3 (case A, B, C below) squares with impunity

Legend: O: available space, X: unavailable space

• Case A: 5-foot square:

  ╔═══╦═══╗
  ║ O ║ X ║
  ╠═══╬═══╣
  ║ X ║ X ║
  ╚═══╩═══╝
  

• Case B: 2 5-foot squares:

  ╔═══╦═══╗
  ║ O ║ O ║
  ╠═══╬═══╣
  ║ X ║ X ║
  ╚═══╩═══╝
  

• Case C: 3 5-foot squares:

  ╔═══╦═══╗
  ║ O ║ O ║
  ╠═══╬═══╣
  ║ X ║ O ║
  ╚═══╩═══╝
  

• Case D: 10 foot square:

  ╔═══╦═══╗
  ║ O ║ O ║
  ╠═══╬═══╣
  ║ O ║ O ║
  ╚═══╩═══╝
  

  Can a large creature finish its movement in Case A, B, or C?

We deal with surface areas not volumes in D&D. If the height of a ceiling is shorter than the height of a large creature, then the DM should apply the squeezing condition regardless of the fact that it fills 10 square feet.

Answer 1

If a creature can squeeze through a space, it can stop there too

Page 192 of the PHB says:

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

Attacks in 5e are distinct compared to movement, that is a creature can move, stop to attack, then move again, but never attack while moving. A creature that is squeezing can still make attack rolls, although they are at disadvantage, so we can infer that the creature must be able to stop in order to make that attack. Since we now know that the creature can stop for an attack, and since there is no other specific rule saying that the creature cannot stop while it is squeezing, then we can infer that in general a creature can stop in any space that it can squeeze into.

Additionally, there is no rule saying that a creature has to end their turn no longer squeezing, nor that they can't end a move while squeezing. This is fairly easy to see in practice if players ever have to squeeze through a narrow cave passage that is longer than their move speed. If a creature couldn't finish their move while squeezing it would be impossible to, say, squeeze through a 200' long narrow cave passage, even though there is no rule that says this is impossible.

A Large creature can fit into a space as small as 5' x 5', though it will be squeezing while it remains there:

As stated before:

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

Page 192 of the PHB lists the various size categories. Large creatures are 10' x 10' large. Thus it can squeeze, and then stop moving, in a space large enough for a medium creature, which is listed as 5' x 5'. Though, if the creature does not have at least 10' x 10' of room available, it suffers all penalties associated with squeezing. So, a large creature can finish movement in all 4 cases that you have provided, though it will suffer all penalties associated with squeezing if it does not have at least 10' x 10' available.

Amorphous creatures don't typically have to squeeze

As Icyfire's answer illustrates, the rule for amorphous creatures follows:

Amorphous. The ooze can move through a space as narrow as 1 inch wide without squeezing.

The ooze can move into a space as narrow as 1" wide and finish moving there without squeezing. Since it isn't squeezing, it suffers none of the associate penalties (disadvantage on attack rolls/dex saves and more expensive movement).

Answer 2

The Ooze does not experience any of the penalties for spaces greater than 1 inch.

The line in question goes (MM 243, for an ochre jelly):

Amorphous. The ooze can move through a space as narrow as 1 inch wide without squeezing.

It doesn't say that it ignores the movement penalty of squeezing, it says that it avoids squeezing altogether. In other words, it can move through such spaces totally normally. Therefore, it does not experience any of the squeezing penalties.

While it looks like the particular wording here only applies to movement on your turn, the wording on squeezing in the PHB seems to suggest that it's referring more generally to passing through a tight space over multiple turns (PHB 192):

You can complete your turn while squeezing

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.

Since you can make saves and attack while squeezing, it seems like squeezing is referring to the general state of being in a small space. It follows that it's possible for creatures to end their movement on squares that are too small for them normally.

Note that creature size doesn't refer to its actual, physical size (PHB 191):

The Size Categories table shows how much space a creature of a particular size controls in combat.

Thus, just because a creature is listed as Large doesn't mean that it literally occupies a 10ft by 10ft space. It might have tentacles or arms that could potentially reach that for, for example.

Additionally, if the feature was referring only to movement, it would probably say something like "the ooze can ignore any movement penalties while squeezing".

Comparison with other creatures

The Yochlol (and spiders) has the following ability (MM 65):

Web Walker. The yochlol ignores movement restrictions caused by webbing.

Note how it is specific to movement restrictions, which means that the creature can be affected by say, burning webs. If the ooze's ability only applied to movement, it would likely have similar wording.