Explaination about why window changes size in different video modes

SCREEN 13 has 320x200 pixels, so its window has 320x200 pixels
SCREEN 12 has 640x480 pixels, so its window has 640x480 pixels

Now, what's the resolution of SCREEN 0? The answer depends on the screen's WIDTH selected and the number of rows. The default is 80x25 with characters which are 8x16 pixels. Doing the math (80*8,25*16) you'll find this mode has a pixel resolution of 640x400.

There are also some aspect ratio issues in full screen mode which I need to address too. At the moment it always displays a 4:3 ratio screen with black bars at the top and bottom if the QBASIC screen mode doesn't have a 4:3 ratio. Ideally it would stretch the screens surface or select a more appropriate video mode. Of course, in window'd mode you'd always expect the window to have the same ratio as the desktops video mode.

320x200 in window'd mode can be a little small. I'll probably add some scaling options and "renderers" (like 2XSAI) at some stage.