From Knoppix installer documentation
If you are going to use the beginner type be aware that you are still using the auto-configuration so you can change your cheatcodes, the flip-side is that changes you may think you are making permanently will be overwritten when you reboot.
I use a debian installation and for the config you are looking, in /etc/X11, edit the file (on root) XF86config-4
The next code is an example from a section of my XF86config-4 file
Code:
Section "Screen"
	Identifier "Screen0"
	Device     "Card0"
	Monitor    "Monitor0"
	DefaultColorDepth 16
	SubSection "Display"
		Depth     16
		Modes "1024x768" "800x600"       <====== screen resol...
	EndSubSection
EndSection
On that section Insert in the mode you want for your screen resol.

I really don't know if this work in your case for the type of installation you choose.