2 hours?? Wow! That's impressive! The concept is neat, and the execution is mostly competent.
There is, unfortunately, a game-breaking error. See, the size limit variable changes with the time the buttons are pressed, not the total increase in block size. That means, if you grow multiple blocks, but then only shrink one block, you can replenish your size limit endlessly, getting arbitrarily large blocks without tripping the limit.
Other than that, great work!