Open the terminal and type lshw. Scroll to the graphics section and post the
results here. It is likely that you are not using a driver that will support
beyond 800 x 600. It is likely that it defaulted to this setting when it
installed as a result of not finding a driver on the CD. To go beyond that
you will have to use another driver.

You can go to System, Adminstration, Hardware drivers and try to download
one if it exists. If one does then it should clear up your problem.
Depending on the results of lshw above, users may be able to assist further
if we can't find drivers in the repositories.