You are correct, there is no way to exactly what you are describing. I think the biggest reason is that there are plenty of situations where large messages on a busy server configured this way might not ever get a chance to go out. It is an interesting problem though, so I'll take any offline feedback about why this feature could be important -- it would make a good discussion piece at least.
Untested workaround: If you have multiple servers, you could in effect configure this using the max size limit on SMTP connectors and some sort of QOS -- but not on a single server.
On a related note: a good best practice is to be mindful of your available bandwidth and load, and then set reasonable size limits. I have seen many a server that there was no mathematical way for the server to push the volume and size of messages through the small network that was available to it.