postgres=#
SELECT pg_database.datname as "database_name", pg_size_pretty(pg_database_size(pg_database.datname)) AS size_in_mb FROM pg_database ORDER by size_in_mb DESC;
database_name | size_in_mb
---------------+------------
DBACLASS      | 7767 kB
postgres      | 7735 kB
template1     | 7735 kB
template0     | 7601 kB
(4 rows)
(or)
postgres=#
\l+
List of databases
Name    |  Owner   | Encoding | Collate | Ctype |   Access privileges   |  Size   | Tablespace |                Description
-----------+----------+----------+---------+-------+-----------------------+---------+------------+--------------------------------------------
DBACLASS  | postgres | UTF8     | C       | C     |                       | 7767 kB | pg_default | TESTING DB
postgres  | postgres | UTF8     | C       | C     |                       | 7735 kB | pg_default | default administrative connection database
template0 | postgres | UTF8     | C       | C     | =c/postgres          +| 7601 kB | pg_default | unmodifiable empty database
|          |          |         |       | postgres=CTc/postgres |         |            |
template1 | postgres | UTF8     | C       | C     | =c/postgres          +| 7735 kB | pg_default | default template for new databases
|          |          |         |       | postgres=CTc/postgres |         |            |
(4 rows)