For small and moderate size databases, I find that the simplest way to
estimate the database size is to do:
du -ks /var/lib/postgresql/data
...create a new database, restore a backup of the database in whose
size you're interested in
du -ks /var/lib/postgresql/data
Compare the first and second "du" output and you get possibly the
exact size of the database.
Cheers,
t.n.a.
