http://www.knoppix.net/docs/index.php/CheatCodes

these cheat codes are also on your cdrom.
look and see what you could do:

at the bootup promp press F2, you now get the cheatcodes you can enter before the total bootup, now for you try :

knoppix xvrefresh=60 screen=1024x786 (or if that fails 800x600) and boot by pressing enter. Happy knoppixing!