You need a fortimail to do that reliable and off the top of my head, I don't know how it can be done for a min in the content body. Typically we set limits on the MAX size of the body but never heard of anyone doing the opposite.
Can you explain why you need todo this? And are you looking to send a bounce notification as to why it was not delivered or just quietly drop the message?
In the mean time, I 'm going to poke around a see just how to drop a message with no message body. I believe it's simple for no subject or limits se on MAX sizes in the mail header or subject line.