Вот собственно запрос:
SELECT table_schema "database_name", sum( data_length + index_length )/1024/1024 "Data Base Size in MB" FROM information_schema.TABLES GROUP BY table_schema;
Вывод будет в такой форме
+--------------------+----------------------+ | database_name | Data Base Size in MB | +--------------------+----------------------+ | information_schema | 0.00878906 | | mysql | 0.61256504 | | performance_schema | 0.00000000 | | stephensmedia | 8.15625000 | | tutology | 3.56811810 | +--------------------+----------------------+ 5 rows in set (0.04 sec)