Thanks, Viktor. I got this partially working by temporarily separating the master.cf entries. Email that is sent to my email server via smtp now gets rejected if >message_size_limit while my mail server still accepts bigger incoming emails. Great!
But outbound email originating on the mail server itself (users sending mail via 'mail -s "test" u...@gmail.com' on the mail server) is still going out, even if the mail size exceeds message_size_limit. My mail server is configured to relay all outbound email to my ISP's mail server via the "relayhost=" directive. Is this bypassing the smptd setting somehow for locally generated email? Thanks much for your help!
