I'm not in Knoppix right now, but are there different resolutions preconfigured in the /etc/X11/XF86Config-4? To find out, press CTRL&ATL&- (minus on the numberic keypad). CTRL&ALT&+ (plus) *should* jump to the next highest resolution configured in XF86Config-4.

To make the changes permanent, delete instances of "1024x768" in XF86Config-4 (and anything higher, for that matter).

-SUO