What size screws do you use to mount it on the post

Hello, the hardware needed to mount the Cambridge 6.8-in W x 8.9-in H Metal Black Post Mount Mailbox is dependent on the post it is being mounted to, which is why we do not include installation hardware for our mailboxes. For all Architectural Mailboxes posts and mounting plates, we provide the following: 5/16”-18 Hex Bolt, 1” Length (4), 5/16”-18 Hex Nut (4), 5/16” Large Flat Washers (8). This hardware will work for standard posts. If you are installing the Cambridge on a 4x4 wooden post, the 7540B-10 or 7545B-10 mailbox adapter (includes necessary hardware) or your own wooden plank will be needed for installation. Thanks for reaching out and please let us know at 310-374-5700 or support@architecturalmailboxes.com if you have any further questions.