You need to use userlibbage. This will help:
http://www.blitzbasic.com/codearcs/codearcs.php?code=829
..particularly this bit:
win\DesktopW = User32_GetSystemMetrics(SM_CXSCREEN)
win\DesktopH = User32_GetSystemMetrics(SM_CYSCREEN)
http://www.blitzbasic.com/codearcs/codearcs.php?code=829
..particularly this bit:
win\DesktopW = User32_GetSystemMetrics(SM_CXSCREEN)
win\DesktopH = User32_GetSystemMetrics(SM_CYSCREEN)