SELECT pg_size_pretty( pg_database_size('dbname') );
/* For multiple databases */
select t1.datname AS db_name,
pg_size_pretty(pg_database_size(t1.datname)) as db_size
from pg_database t1
order by pg_database_size(t1.datname) desc;
SELECT pg_database_size('named_db');
SELECT pg_size_pretty(pg_database_size('named_db'));