The best way to test things out, is by opening up a second tty (Ctrl+Alt+F2), and shutting down the X server (as root):
$ cd /etc/init.d/
$ ./xsession stop
This will shutdown the X server
After that, edit /etc/X11/XF86Config-4 to change the resolution. It's best to change it in every display depth (in case Knoppix decides to use another pixel depth).
After that, do a simple 'startx' to get back into X.