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 (1×1) to large size (2×2)? 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 2×2? 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?
Best Answer
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.
For the adjudication, as Rykara mentions in the related answer
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):
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
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.