is there any way to change Knoppix by running it (instead of using chroot, which makes difficult to change KDE settings, for example)
Have you seen the HOWTO that I wrote? http://www.knoppix.net/forum/viewtopic.php?t=13867 It documents how to change KDE settings in the chroot.

As for ISO size. A simple fool-proof method does not exist.. Approximately 1835MB is compressed down to a 696MB cloop image file. I exit the chroot and do a
Code:
du -h --max-depth=1 /mnt/hdb1/remaster/
to get a total size. If it is under 1850MB then it should create a compressed loopback image file that will fit on a 700MB CDR.