The CD is designed to work at 800x600, but the HD installation can work at the maximum detected by the Knoppix auto-configuration system. Unfortunately, it doesn't always recognise the video hardware, so it drops to a safe VESA mode.
That's exactly what happened when I installed it on my new notebook.
To fix it, I edited the xorg configuration file: /etc/X11/xorg.conf
Take a look at the following section, to find out what screen modes have been detected:
...
#
# Auto-generated by KNOPPIX mkxf86config
Section "Monitor"
...
Find a suitable mode, then add it to the 'Screen' section a bit further along. On my computer, this only had resolutions for 800x600 and 640x480, so I had to add 1024x768 to each line.
Section "Screen"
...
SubSection "Display"
Depth 16
Modes "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 32
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Save the file, and reboot. Now in the Amiga screenmode prefs, you should have the higher resolution(s) available!
I hope that helps...
Amir