psql:
SELECT pg_size_pretty(pg_database_size(pg_database.datname)) AS size_in_mb,
pg_database.datname as database_name
FROM pg_database ORDER BY pg_database_size(pg_database.datname) DESC;
With index use: pg_total_relation_size, insead of pg_database_size
size_in_mb | database_name
------------+---------------
7571 kB | template1
7499 kB | postgres
7345 kB | template0
(3 rows)