Monitors are 14", 15", 17" etc. What you are refering to is the screen resolution and that is determined by your video card. If your card can't handle higher than 640x480 it is time to get a new(er) one or if you are lucky you can add extra ram to the card (+/- Euro 2,50)
Why not just change the screen resolution to 800x600?
The size of the skinless interface is roughly 540x570. The skinless interface can only be enlarged.