Great concept! I liked it a lot.
If you decide to continue I would suggest adding some more downforce when the player is falling, since that floating in the air felt a little too much. Also, I feel like an immediate size switch (without a timer) would have been nice to have, or at least some kind of indication on the cube that F can not be used right now.
Overall, cool idea and nice execution, levels were also great with challenging stuff. I hope you continue to work on the game in the future!