This can be very helpful to know, for various reasons. You will need to be logged into your dedicated server or VPS server via SSH, as this command will require you to be logged in via SSH.
du -ha /home/john | sort -n -r
Replacing "/home/john" with the directory for which you want to know the disk space used for that particular directory. You will see an example output like below
root@localhost[~]# du -ha /home/john | sort -n -r
228K .
64K ./mail
56K ./.cpanel
28K ./tmp
28K ./.fantasticodata
28K ./.cpanel/datastore
24K ./.cpanel/caches
16K ./mail/.Trash
16K ./mail/.Sent
16K ./mail/.Drafts
12K ./.cpanel/caches/filesys
8.0K ./tmp/cpbandwidth
8.0K ./public_html
8.0K ./public_ftp