As far as I know "attachments" aren't separate entities so for limiting the size of attachments you'd have to resort to tweaking "MaxMessageSize" in sendmail.cf/submit.cf. If you don't mean size limit but amount of attachments or deny attaching stuff then you probably have to resort to filtering before messages gets submitted. For an example that doesn't use Milter (if there are any) you could look at
http://memberwebs.com/nielsen/ 's ClamSMTP and combine it with formail+procmail.
