I'm using the SMTP proxy for relaying mail to our internal mail system. I would like to set the message limit to a specific size, how do I alter the mail size limit?

You need to create a file named "databytes" in the /var/qmail/control/ directory.

Inside the databytes file, you need to specify the value, i.e 200000 = 200K bytes, 2000000 = 2Mbytes