About 1GB should be more than enough. But you may need some place for persistent store, too. (And that store will be used if you copy the whole KNOPPIX directory to an HDD partition. That may be Klaus K's explanation that you await: Why run from CD when you already use HDD real estate for Knoppix files? In that case, you only boot from the liveCD, you don't run from it - and your drive isn't occupied either.)