Try and check out what has been left out in other remasterings:
1) Of Knoppix 6.0.1
2) Of earlier versions
Also check the size - big chunks like OpenOffice really helps, small packages may be left in until you are sure about them.
I think you have to define the "backbone of Knoppix" yourself. I doubt Java is needed for getting it up and running, for example. But it may not be the best thing to leave out, though.. For my use, that is YMMV

By the way: What is your size goal, and why? Using mostly USB pendrives, I don't feel that need to strip down the basic install, anymore.