I have gentoo, but I think the idea is the same:
You can edit the xf86config file to display different modes. Here's how you do it:
In the file you'll see a few lines like this:
To change it, edit the file (not sure in debian but in gentoo the command is nano) and where it says modes replace the "1024x768" with "800x600". You can delete the other modes so that it won't automatically pick 1024x768.
Lemme know if this works.