If you check the display-relevant parts of your XF86Config-4 file (or post them here), you may find the spot where Knoppix is misunderstanding your monitor. Hard to guess without looking at the configuration.

Meanwhile, the quick-and-dirty way to get around the problem is to adjust screen resolution once you're in X. Do Ctrl-Alt-Plus Sign and/or Ctrl-Alt-Minus Sign (using the + and - on the keypad, not the ones to the left of Backspace) to scroll through the resolution options defined in XF86Config-4.

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