On my server I generally use "du -h --max-depth=1" but some isp's, for whatever reason, don't allow you to use the command.
Generally you'd do:
cd /var/www/html/ubb (non-cgi path)
du -h --max-depth=1
It'll show you how large every directory under than path (noncgi path) is. You can additionally do this for the cgi-bin path but nothign really changes there to be a sizable addition to your used space (mainly the cache and forum files).
