database_size

-- PostgreSql
SELECT 
  pg_database.datname AS "Database Name",
  pg_database_size(pg_database.datname) / 1024.0 / 1024.0 AS "Database Size (MB)"
FROM pg_database;

-- MySql
SELECT
  table_schema "Database Name",
  sum( data_length + index_length ) / 1024 / 1024 "Database Size (MB)" 
FROM information_schema.TABLES 
GROUP BY table_schema;