individual mail message size "message too large"

I have not been able to find this information via the forum or google, so I apologize in advance if I missed it.

Where can I modify the individual message size for a user sending email?

It seems to be set at about ten megs, and unfortunately users now a days are sending messages larger then that. I set the quota on the users mailbox to 500 meg, so it must be somewhere else.

Thank you