You would have to accept the message in the HAT and then use an Incoming Mail Policy to select recipient addresses. Add a content filter to that policy to select on size.
Since it sounds like only a few users should be able to recieve large messages you could use two policies.
1) Large message recipient policy
message size < 30mb, Action=DeliverNow
2) Default policy
If message > 10mb, Action=Bounce
You just can't bounce the messages right in the SMTP conversation is the only real limitation.