#dpkg-reconfigure xserver-xfree86
Take the Advanced option, enter your horiz and vertical frequencies, mouse and it will give you a list of resolutions, you can choose the one you want.
For a step by step explanation of the process:
Steps 9 and 10
The Very Verbose Debian Installation Walkthrough
http://osnews.com/story.php?news_id=2016
Also check the sticky: Debian Configuration Post-Install in this forum.