It took me a bit to find out how to do it, so I thought I'd post it here out of usefulness. It's super simple though. To change the screen resolution (the size of the game window) in RPG Maker VX Ace, copy and paste this line into line 6 of the script called "main":
Graphics.resize_screen(width, height)Replace width and height with the size of your screen. You can't go above 640 by 480 (which is larger than the default screen in VX Ace).
Handily, the game's designed for this, and all windows and such resize accordingly, although a little awkwardly when you get to very small sizes (but you should know that anyway).
You can also call this at any point during the game to change it's size, for whatever reason.