It forces your screen to 640x480 mode, which makes the graphics bigger. The game's graphics are 608x392, which will use about 75% of the screen at this resolution (less if you have widescreen). If you use a bigger resolution, the game's graphics will still be 608x392, and will be smaller.
Though, if you use GOG's default compatibility settings, they're forcing the screen to 640x480 mode anyway, so in that case it doesn't really matter whether you have it on or off.
Post edited December 22, 2009 by ikantspelwurdz