Start with the good: the music choice and art style was great. Really reminded of the 16-bit platformers I used to play like James Pond. The shrinking and growing mechanic was an interesting addition (would have liked to have seen more done with it but I liked it).
The main complaint is the jump; it's extremely inconsistent. I'd estimate 1 in every 3 attempts to press jump just didn't do anything, which isn't ideal in a platformer where you have to time your jumps.
Character controllers are notoriously hard to do well so it's a completely understandable problem, just unfortunate in this type of game. Good job!