How do you limit the max size passed to SA

[R. Steven Rainwater]

I've just switched from smtp-vilter to spamass-milter and I can't figure
out how to set the maximum message size passed to spamassassin. We have
a very high volume mail server and it's important to have a cap on the
size of emails being checked by SA so we can keep our server load under
control. I've checked the man page but can't figure out how to do it.
Can anyone help me out?

-Steve