Requête

SELECT table_schema AS "Database name", SUM(data_length + index_length) / 1024 / 1024 AS "Size (MB)"
FROM information_schema.TABLES
GROUP BY table_schema

Résultat

+--------------------+--------------+
| Database name      | Size (MB)    |
+--------------------+--------------+
| database 1         | 607.70312500 |
| database 2         | 647.92187500 |
| information_schema |   0.00878906 |
| database 3         | 417.16526890 |
| database 4         | 567.17187500 |
+--------------------+--------------+