Here is shell a trick that I used lately.
Before moving to new system (where I enforce quotas on all home folders), I had portable home directories for all the users on my shared server storage. The old setup did not strictly apply the quota limits on all users. Some of the users had large home folders and a couple of them logging on different systems, would fill up the whole hard disk on that system.
To find out how much space each user was using on the server, I ran the following command in their home directory.
$ du -chd -1 > $HOME/user_space_report.txt
Simple.