Try adding your desired screen resolution (example: "1024x768") to /etc/X11/XF86Config-4 as root and reboot. This worked for me with Debian "Sarge" after doing net install. You may even need to run the following commands as root after editing this file:
Code:
# cp /etc/X11/XF86Config-4 /etc/X11/XF86Config-4.custom 
# md5sum /etc/X11/XF86Config-4 > /var/lib/xfree86/XF86Config-4.md5sum
# dpkg-reconfigure xserver-xfree86