Help with hitbox of custom mob

Hi, im new in Mcreator and i was wordering if there's a way to change the hitbox of a mob in-game, like the player does;
When you sneak, the player hitbox reduces it's Y, making possible pass trough blocks that not any mob can do. I was wondering if you can do the same thing with a custom mob, like 1x1 spaces and then return to its original hitbox state. 

Any answer will come in handy!!! :D