We use PostgreSQL for SpatialKey and because of that I have learned a lot about system administration with PostgreSQL. While debugging an issue with tables that had ballooned up I ran into this article which has some great tips for isolating db and table size with a few quick commands. http://www.thegeekstuff.com/2009/05/15-advanced-postgresql-commands-with-examples/

Also check out 15 Practical PostgreSQL database administration commands and backup and restore PostGreSQL with pg dump and psql.