You can use below query to find the size of all databases of PostgreSQL.
Reference is taken from this blog.
SELECT
datname AS DatabaseName
,pg_catalog.pg_get_userbyid(datdba) AS OwnerName
,CASE
WHEN pg_catalog.has_database_privilege(datname, 'CONNECT')
THEN pg_catalog.pg_size_pretty(pg_catalog.pg_database_size(datname))
ELSE 'No Access For You'
END AS DatabaseSize
FROM pg_catalog.pg_database
ORDER BY
CASE
WHEN pg_catalog.has_database_privilege(datname, 'CONNECT')
THEN pg_catalog.pg_database_size(datname)
ELSE NULL
END DESC;