We had an issue where the rackspace intelligence monitor was giving a different value to the control panel instance of a dbaas. So I came up with a way of testing MySQL for the database size. There is nothing more reliable than running the query like this, I think.

SELECT table_schema AS "Database", 
ROUND(SUM(data_length + index_length) / 1024 / 1024, 2) AS "Size (MB)" 
FROM information_schema.TABLES 
GROUP BY table_schema;