While in an X session you can temporarilly change resoloutions and size by the keyboard combination:
CONTROL + ALT + <keypad>+[or]-
In your /etc/X11/XF86Config file you can add and tweak these options:
DisplaySize 255 190
[NOTE - refresh rates can by found in your monitor manual or possibly at the SourceForge page for this or you manufacturer's
site. That "DisplaySize" option is great if you can find that answer for you monitor.]
[NOTE - adding this option will force X to open with your preferred depth and mode (modes are read in decending order).]
Modes "1024x768" "800x600" "640x480" "640x400"
Virtual 800 600
[NOTE - under the SubSection "Display" above you can add the options "ViewPort 0 0" [this would start the upper left of your
screen at 0 0 [x y] - if you *omit" this "ViewPort" option from your configuration, it will default to _centering_ your
display on the screen] and "Virtual 800 600" (the numbers are your choice of mode to start in, limiting the
virtual/scrollable area to your screen size or not)].
I hope this makes sense. I had written this all earlier, but my login here timed out during answering and I lost it all and
dont' have quite the same patience to repeat it. :)