i like the concept, very solid and can lead to an exciting precision-platforming puzzle game. however it feels pretty misleading to teach players that each color related to a certain size ball, and then immediately go back on it with level 3 telling players that yellow is now random. i think random size platforms could be a fine mechanic if indicated by a different color, but then again it also feels a bit unfair to the player to make success random in a precision movement game.
the size controls could also be easier to pick up if they were in line (a, s, d for example), or configurable even.
the collision could use a bit of polish as well. I was able to move through walls as the ball grew even though it was the incorrect size for that color. it looks like you're using a box collider on a circular character, which is a little visually confusing to me. i could see a circular player collider being difficult to work with so maybe try out a square player.
one last minor thing, in the tutorial i fell off underneath the z platform and the level didn't reset. i ended up having to reload the page.