Here's what I once wrote (it was awhile ago, please be nice, but feel free to criticize :) so I could say 'dirsz *' and get a sorted list of directories and sizes (I would just point you to a page on IIUG where it resides, but it seems to be down right now, so I'll post this for now & remove it later):

Update:deleted the code here, it is in the upd_stats bundle in the DBA tools section of IIUG.