..to do all the system update/upgrade/app installation..
For that all I think you need at least 5-10 GB free space in your persistent memory.

But be aware you may run in trouble, if you try to update Knoppix and you will get serious problems, if you upgrade it except when you are a very experienced user.

Greetings Werner * http://www.wp-schulz.de/knoppix/summary.html
Own Rescue-CD (Knoppix V6.7.1 remaster)