If you run Windows XP, just right-clic on your Desktop (on the wallpaper where lie Your computer and Trash icons), in the then opened window, select the last thumbnail called Settings.
At the bottom, there's a slider called Screen Resolution. Pull it to the right to increase (up to the desired 1920x1080, normally recognized by the video card —ATI 9800 ?), to the left to decrease your screen size/resolution.
If maybe you've upgraded to Vista or Seven, (I never used these OS) it should work the same way if doesn't work, please see here, a different way explained on MS website.
If maybe you've plugged it with the VGA cord (blue tips), you should press the Auto adjust button in the front of your monitor —useless with DVI cord.
Hope this helps.