SELECT pg_size_pretty(pg_database_size('name of database'));
Will give you the total size of a particular database however I don't think you can do all databases within a server.
However you could do this...
DO
$$
DECLARE
r RECORD;
db_size TEXT;
BEGIN
FOR r in
SELECT datname FROM pg_database
WHERE datistemplate = false
LOOP
db_size:= (SELECT pg_size_pretty(pg_database_size(r.datname)));
RAISE NOTICE 'Database:% , Size:%', r.datname , db_size;
END LOOP;
END;
$$