The good news: this totally worked to give me screen size options for some games in Ubuntu.
The bad news: I didn't take into account the menu bar at the top of the screen, and the resized game doesn't allow me to move the screen up high enough to access the "OK" button at the bottom of the screen that would let me change the screen to an even more appropriate size. So now I have a game that runs in a screen about 10% larger than the actual screen, and no way of actually clicking "OK" to confirm a smaller screen size. There doesn't seem to be any way to tab to the OK button, or to click it "below-screen" -- the mouse stops at the screen bottom.
Is there a way to force a program or game to start at a certain resolution?