The standard Mach3 screens need at least 1024x768. You can download the screen designer, and try resizing and/or moving items around to fit your screen.
I noticed there is a 640x480 screen available from the ftp site at www.machsupport.com. The FTP is at the bottom of the page. Once loaded, it's under Mach3>Screen Sets.
This 640 x 480 screen may not be current or have all the options the release version screens have, though.
Is the problem that the monitor won't support that resolution, or the video card or built in video won't support it? I've seen some 15" LCD's that use 1024x768 as their native resolution.