mysql> select table_schema, sum((data_length+index_length)/1024/1024) AS MB from information_schema.tables group by 1;
+--------------------+-----------------+
| table_schema | MB |
+--------------------+-----------------+
| prod | 298025.72448921 |
| information_schema | 0.00781248 |
| maatkit | 70.77330779 |
| mysql | 0.66873168 |
| test | 4752.31449127 |
+--------------------+-----------------+
5 rows in set (0.01 sec)