since your going from a 1x1 to a 2x2, what happens in this situation?


EEE
EAE
EEE

E=enemy A= ant-man

since giant man would take more space, how would this resolve?

Grow isn't allowed
enemies is scares were giant man are knocked backed
?????

Core Rules page 7, The Rule of Occupancy:
Any game effect resulting in two or more characters
occupying the same square, or characters, terrain
markers, or objects occupying a square of blocking
terrain, is prohibited (except for Debris markers,
see Terrain, p. 12). If part of a game effect would
cause this to happen, that part of the game
effect is ignored.

So you couldn't perform the action.

spacecase2035 is correct. You cannot perform an action that would create an illegal game state.

In some other rules (Golden Rule 3, that say that can be placed in a different square.) but i prefer orange confirmation

Since replacing a character with another character is neither placing or moving, we go with the first part of the Rule of Occupancy which says:

If part of a game effect would cause this to happen, that part of the game effect is ignored.

So you can still give Ant-Man a move or close combat action that deals no pushing damage, but because replacing him would brake the Rule of Occupancy, you ignore the replacing.