How does increasing in size affect adjacent squares and enemies?

Inspired by this question and effectively asking the opposite question. Assuming play is on a grid, what squares can a creature occupy when it goes from being medium size (1x1) to large size (2x2)? Does the square it already occupied when it was medium need to be included in its new form? What are the options for the three additional squares, can they simply be any that would make the creature 2x2? How does increasing in size interact with the "Moving Around Other Creatures" rule which states:

Whether a creature is a friend or an enemy, you can't willingly end your move in its space.

So if there were a creature in one direction would you not be able to include its current space in your new form/size? What if you were surrounded by creatures, could you increase in size at all?

Some hopefully helpful diagrams: You are C, monsters are X, empty spaces are #.

Can

###
#C#
###


change into:

CC#
CC#
###


or

#CC
#CC
###


What can

XXX
#C#
###


change into?

What can happen from this last scenario:

XXX
XCX
XXX


Note: I am looking for an answer that is rooted in RAW, but if no answer exists there an answer from experience with this issue would also work.

Examples of why this might matter:. If you end up occupying the same space as an enemy then a spell like fireball would no longer be able to target you.
If you push the creatures out of the way this could do things such as pushing then into a moonbeam spell.
If you are not allowed to occupy the same space as the monsters then I am confused what would happen if you were initially surrounded.

What squares can you occupy when your size increases, and do other nearby creatures impact this?

• @Medix2 they are bad for screen readers for non-code content. Here, I'd say it is as close to code as you get on rpg Q&A, and reading "letter by letter" is something you actually want for blind people, so code block is appropriate. – Mołot Aug 12 '19 at 21:06

Up to the DM

This is about it. As far as I am aware, the rules do not specify how the creature will occupy the space after being transformed. In particular, there are few specifications on how to play using miniatures/grid on the core books.

the grid space of a map does not represent the actual space a creature occupies, it represents the space that creature controls

so, there are a few things to consider: First, enlarging a creature might not require it to actually occupy the space suggested for a larger creature, which is also backed by the DMG (p. 251):

For example, you might use a miniature that has a Large base to represent a Huge giant. That giant takes up less space on the battlefield than its size suggests, but it is still Huge for the purposes of rules like grappling.

Second, if you are using the optional "facing" rules (DMG, p. 252), it would make sense that the space the creature controls is defined by the way the creature was facing.

Finally, for your scenarios where the creature is, for example, surrounded, it would make sense that the creatures are pushed back, if the creature growing is in fact enlarging enough for it to occupy other spaces. Also, on your quote, note that

Whether a creature is a friend or an enemy, you can't willingly end your move in its space.

This means creatures can occupy the same space, they just can't do it willingly (in particular, they can't willingly enter other creature's space by moving and stay there). Thus another possible adjudication in this case (surrounded creature) is simply allowing them to occupy the same space and, in the monsters' turns, they are forced to move out of the space (as they can't willingly stay there). (The character doesn't have an option unless the monsters are Small or smaller, since he can't move through the monsters).

From my experience1, ultimately none of this should have a relevant impact in the game, so you can adjudicate as you want and it should be fine if this situation shows up in an actual game.

1 Just for reference I have played lots of wizards with Polymorphs and Enlarge/Reduces in online tables (with grid/miniatures) and I can't recall one time when the occupied spaces would have been game-changing. The DM/I just increased the token's size as we saw fit and moved on.

• So in the scenario where one side is walled off by enemies would the creature be allowed to push them out of the way to occupy their spaces? Perhaps they could occupy the same spaces (it is unclear to me if that counts as "ending their move" hence my question here)? Or would the creature be required to have their new squares be a set of the ones that do not include the enemies? – Medix2 Aug 13 '19 at 0:31
• If by walled you mean your fourth scenario, you can: push the monsters away; be forced to move away (1 square behing, occupying a free space, possibly allowing opportunity attacks in this case); or occupy the same space and possibly the monsters be forced to move later (without an opportunity attack as they are not leaving the creature's range). Again, except for the one that forces you to take opportunity attacks, any of these shouldn't have a relevant impact in the game. – HellSaint Aug 13 '19 at 0:41
• Note: it is impossible to require that the creature occupies a set of squares that is not being occupied by enemies in your last scenario, so I don't see this adjudication as making much sense (no reason to believe surrounding someone would stop them from being Enlarged by a spell, for example). – HellSaint Aug 13 '19 at 0:42
• I have edited my examples above into my question, if you'd rather I roll back the edit I can do so, but I believe they express my original concerns – Medix2 Aug 13 '19 at 0:54
• @Medix2 I don't think you will find a RAW answer since playing with grid isn't considered in RAW (everything in the books about miniatures/grids are not "RAW", they are just guidelines for adjudication to help the DM). Whether or not an area of effect spell affects a creature is, by design, left to the DM to adjudicate. Besides, of course there are examples where this could matter - the fight could be happening in a cliff and pushing the monsters would kill them. After 5 years playing (this edition), I haven't seen any of these cases. – HellSaint Aug 13 '19 at 18:09