On which level ?
(Btw, level 1 is tha hardest, so start with level 2, then infinite, then level 1; this will be fix next update)
You need to left click / right click to change your size. When you're bigger, you're slower but jump heigher, and it's the other way around when you're small.
Btw, the levels aren't finished at all because they got corrupted when we gathered our work ( with the 2 other devs), so they're not finished.
But next week, we will update the game, to put / fix things which got corrupted; we already have the update ready !