Virtual screen? You mean where the "desktop" is larger than the physical screen and you scroll around it?
Assuming that's what you mean, just edit your xorg.conf and add a "Virtual" line to your screen section. For example, you might have something like
Code:
Section "Screen"
...
DefaultDepth 24
SubSection "Display"
Virtual 1600 1200
Depth 24
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
EndSection