[Mobhack]

The free demo game can only be played in 640 or 800 resolutions, whether windowed or full screen modes.

If you want to play in the desktop's resolution or one of the larger 4:3 windows resolutions, then you'll need to purchase the full game. (I'm currently happily playing on a 1080p 1920x1080 monitor).

However - in the meantime you might try setting your monitor to a lower desktop size and then playing in windowed mode or full screen.

cheers
Andy