This would be a nice feature but hard to implement since MySQL doesn't have any kind of build in quota management. As far as I know the only way to do it, would have some script get a list of all databases per user, tally up the size of all tables within those databases, then do whatever depending on their sizes.
This brings up a question. If a user goes over the limit, what will happen? Suspension? seems a bit harsh, or would it act the same way as quota does now?
Eitherway, personally I won't be limiting users mysql databases since it's included as part of their account quota.
Just my thoughts