mysql> SELECT table_schema "database_name",
    -> sum( data_length + index_length )/1024/1024
    -> "Database size in MB" FROM information_schema.
    -> TABLES GROUP BY table_schema;
+--------------------+---------------------+
| database_name      | Database size in MB |
+--------------------+---------------------+
| cacti              |          0.30696487 |
| exim               |          0.01589584 |
| information_schema |          0.00878906 |
| mysql              |          0.63503933 |
| performance_schema |          0.00000000 |
| roundcube          |          0.48437500 |
+--------------------+---------------------+
6 rows in set (0.01 sec)