Now, I see that my desktop running rhel 5.6 has run out of space. Now I need to clean it up. There must be some unintentional files adding up. What I would like to be able to do is list the files in descending size order in a clean way and see the culprits.
First thing to try is
yum clean packages. Have a look at
man yum for more cleanup options.
Other things you could do are mostly cleaning your own downloaded files, logs etc., to which there' no general rule.
I usually use ncdu for similar purposes.