What size mailbox works with this mailbox post?

The limiting factor is the gap between the platform and bracket. The mailbox needs to be thin as possible at the back bottom side.

A standard mailbox would work. The support brace for this mailbox post is 17 and 3/4 inch length that which holds the mailbox. So if you have an bigger mailbox you'll have to use a support brace between the metal bracket and support platform to attach mailbox.