How to list PostgreSQL database sizes

Use the psql shortcut \l+ , like this

postgres=# \l+ gld
                                                             List of databases
 Name |      Owner      | Encoding |  Collate   |   Ctype    |           Access privileges            |  Size   | Tablespace | Description
------+-----------------+----------+------------+------------+----------------------------------------+---------+------------+-------------
 kml  | klm_master      | UTF8     | en_US.utf8 | en_US.utf8 | klm_master=CTc/klm_master             +| 1448 MB | pg_default |
      |                 |          |            |            | klm_consumer=c/klm_master+             |         |            |
      |                 |          |            |            | klm_producer=c/klm_master+             |         |            |
      |                 |          |            |            | klm_migrator=c/klm_master+             |         |            |
      |                 |          |            |            | postgres_exporter=c/klm_master         |         |            |
(1 row)

By using SQL:

postgres=# select pg_database_size('klm');
 pg_database_size
------------------
       1518559747
(1 row)

# convert to GB:
postgres=# select pg_database_size('klm')/1024/1024/1024 "gb";
 gb
----
  1
(1 row)