Determining size of a database before dumping

Hi all,

   I am (re)writing a backup program and I want to add a section for
backing up pSQL DBs. In the planning steps (making sure a given
destination has enough space) I try to calculate how much space will be
needed by a 'pg_dump' run *before* actually dumping it.

   Is there a relatively easy way to do that? Moreso, if it possible to
do this from an unpriviledged account? If not, is there a way to add the
permissions to a specific pg user to allow that user to perform this?

   Thanks in advance!

Madison