iRedAdmin-Pro-1.3.0 now supports per-domain quota size limit. You can check it out or help test in our demo: http://www.iredmail.org/admin_demo.html (Choose development edition)

Below are screenshots:

You can see quota status, includes quota and account limit, as progress bar in domain list page:

While creating new user, you can see numbers of spare domain quota size and accounts.

Of course you can change quota and account limit. in domain profile page, choose "Advanced" tab:

Enjoy