I think it's an interesting, original concept; well-executed. I definitely did enjoy the game.
The levels are fairly straightforward, I didn't think any of them were particularly hard to figure out, but solving them in practice might have taken some time. You could consider adding new 'puzzle' levels, or, if it makes sense, action levels (i.e. moving environment or enemies would force you to act faster or more precise).
I had a slight issue with the controls. My 'jump' sometimes wasn't registered even when I was on the ground and I'm not sure why. It's also somewhat common in platformers to add tolerance for pressing jump a bit too soon (e.g. ~0.2s) before you actually touch the ground, or after walking off a ledge, for cases when the player doesn't hit jump at the exact frame.
And since the entire game can be played with keyboard except for the one horizontal size slider, I'd consider adding keys (Q, E…?) for manipulating the slider without a need for a mouse.
The character can also stick to walls if you move towards them while in mid-air – if this is unintentional, consider giving something like a PhysicsMaterial2D with zero friction to the character to avoid this to an extent.
Still, for a game jam game, pretty impressive! An original idea with a proper beginning and end, albeit slightly wonky at times.