Create /var/qmail/control/databytes and then within the file specify the maximum size of the message you want to allow in bytes. If you want to limit messages to roughly 10 megabytes then you would put 10000000 in the file, or you can use google so you can get the size right on the money.

# chmod 644 /var/qmail/control/databytes
# chown root:qmail /var/qmail/control/databytes
# qmailctl restart