The reason it's limited is because it causes instabilities. The larger you get, the larger the forces are, and with the walls, it gets.... laggy. Too big and it crashes not only the game but your computer. I chose the max size per-level based on how large you could get without this happening.
I also made the last level the way it is for a reason, for all you size demons who want to just fill up the entire screen.
In terms of altering these values after the fact, there's probably a way to do it, but would require hex editing or Melon Loader, both of which I have 0 experience with