The screen size is usually a fixed height and width > resolution, and you set this in c-script/lite-c using...

var video_mode = (1-11);

1 = 320x200
11 = 1600x1200

I seriously dont recommend using anything below 7 if u want nice results.

To set the screen to window mode or fullscreen...

var video_screen = 1; // fullscreen
var video_screen = 2; // windowed

Search the manual for more info...