in the rules it said we are not allowed to update the game during the voting process but does that mean just the game or the game and the page (like the window or the description) can I change the window size for my game?

Yeah, I would think so because you are not allowed to change game play or mechanics but that seems fine.

you're not allowed to update the gamefile itself (unless you are fixing game-breaking bugs or adding a WebGL version), but you are allowed to change the page as much as you want during the voting process. So yes, you can update the screen size