mailbox size management isn't really a function of sendmail. Using quoatas (man quota) and/or tweaking the local delivery agent (by default rmail) you can change the behavior of sendmail to queue or reject mail for full mailboxes. Most people would replace rmail with a smarter LDA like procmail.
The techniques can be found on the comp.mail.sendmail newsgroup.
Truncating mail would require that you lock the mail box and filter out mail over a certain size. This would be a scripting exercise.
