When you load knoppix there will be a boot splah screen and a prompt. Here you can press F2 or F3 for options.
At the boot prompt you need to type a boot parameter and type enter to continue, try these.
knoppix screen=1024x768

xmodule=vesa

fb=1024x768

Only type 1 line and see which works. Change resolution if monitor and graphics card cannot support 1024x768