Resolved, but this is implemented very very differently.

Doing it by filesize is going to be unreliable (databases don't clean out garbage always), and database-vendor-specific.

Doing it by filesize or number of rows is not what a lot of people will want anyway. Most people probably do want it done by days so they have a certain amount of back-data.

DB quota is not an official MySQL feature.

So instead I've added a new health check that checks database size. This is MySQL-only. You can configure how big you want to let the database get.