Welcome to LQ!!
We'll need some details on your hardware---eg video card and monitor.
Also, give as much detail as you can about what is happening. For example, does the Gnome desktop correctly fill the screen?
Finally, open a terminal and look at the xorg config file thusly:
more /etc/X11/xorg.conf
Post the sections that are similar to this:
Code:
Section "Monitor"
Identifier "My Monitor"
HorizSync 31.5 - 79.0
VertRefresh 50-70
EndSection
Section "Device"
Identifier "** ATI Radeon (generic) [radeon]"
Driver "radeon"
EndSection
Section "Screen"
Identifier "Screen 1"
Device "** ATI Radeon (generic) [radeon]"
Monitor "My Monitor"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1600x1200"
ViewPort 0 0
EndSubsection
EndSection