I like the concept of only being able to attack specific enemies when you're a certain size. It was a little hard to wrap my brain around the control scheme (maybe making "u" the default attack button could be rethought), but I can see it becoming a fast-paced action platformer in the future. The character's movement also feels a bit too floaty, which affects the platforming and causes you to get stuck within enemies sometimes.
With a little more fine tuning, I think this game has the potential to be pretty great! Great work on the sprites and animations btw :)