postgres/contrib/dbsize

This module contains two functions that report the size of a given
database or relation.  E.g.,

SELECT database_size('template1');
SELECT relation_size('pg_class');

These functions report the actual file system space.  Thus, users can
avoid digging through the details of the database directories.

Copy this directory to contrib/dbsize in your PostgreSQL source tree.
Then just run make; make install.