The way window size works right now is it includes the window borders when you set resolution. So if I set window size to 320x256 in Windows 7 it'll fit the window borders inside that resolution so the actual game window is only 304x218. Problem is, window border size varies between OS'es, Windows versions, and theme settings so it's impossible to make sure the game always runs at the same resolution in windowed mode. If I run the same 320x256 res in classic mode for instance, the game resolution is suddenly 312x218.
It would be great if we had an alternate window size setting that made window borders expand to fit the game resolution instead of the other way around.