Quota on the users Maildir

[denpetrov]

    I have Red Hat 5.2 running qmail and I am thinking about setting the
limit on the users "Maildir" I was thinking using quotaon command to do it.
But I can not get it right .(for quota.conf I do not have example so I am
clueless how to configure it).
My question is how do I setup limit on the users maildir ?
Thank you
Denis