Get the size of all PostgreSQL DB

I would like to know all the sizes of all the PostgreSQL DB running on a particular system. For example, in a system I have installed PostgreSQL and created multiple databases(mydb_1, mydb_2, mydb_3...mydb_N) in that. SELECT pg_database_size('db_name'); The above only giving me size of the specific db passed in the sql as parameter. Is it possible to get the sizes of all the databases in the system using single SQL statement.