Assuming you downloaded an .iso, there should have been a file, knoppix-cheatcodes.txt.

When you boot up knoppix, you have an opportunity to enter these cheatcodes. As I recall, you should see

boot:

Try knoppix vga=normal screen=1024x768 or screen=1280x1024

Let people know if it works for you. Myself, my eyesight is poor enough I need 800x600.