I use the following in /etc/X11/xorg.conf to set the resolution and virtual screen size. However, if you use something like Compiz, those settings may override the xorg.conf settings.
Code:
Subsection "Display"
Depth 8
Modes "800x600" "640x480" "1024x768"
Virtual 1024 768
EndSubsection
Subsection "Display"
Depth 16
Modes "800x600" "640x480" "1024x768"
Virtual 1024 768
EndSubsection
Subsection "Display"
Depth 24
Modes "800x600" "640x480" "1024x768"
Virtual 1024 768
EndSubsection
Subsection "Display"
Depth 32
Modes "800x600" "640x480" "1024x768"
Virtual 1024 768
EndSubsection