Hi pankepasa,

You should read the cheat codes in the docs section to learn more about various the various startup options. This is the short answer:

1. At boot press the F2 key and you will be presented with the KNOPPIX cheat codes.
2. Enter "knoppix screen=800x600" without the " "
3. Voila, you have 800x600 X in all its glory..

You may wish to examine the cheat codes closely as you may wish to invoke other options as well say a wheelmouse which is quite handy. You could invoke both options thusly

"knoppix screen=800x600 wheelmouse"

All the best, Dan