For example, you need to find out the email accounts that are using the most space up.
To sort the directories under the home partition in descending size order ...
du --si --max-depth=1 /home/ | sort -n -r |more
du = show disk usage
-si = in human readable (aka "nice") numbers
--max-depth=1 only show this and 1 subdirectory deep
the output is then piped to sort in numeric order reversed (i.e. largest first)
the output is then piped to more so you get it page at a time.