What size screws are needed for this? It would have been helpful if they were included. Thank you.

Mailboxes normally do not include the mounting hardware because there are so many various styles of mailbox posts on the market - including wooden, steel, plastic, aluminum, etc. Therefore, providing hardware for all or even most applications is impossible. (Note: mailbox posts typically include the hardware designed to properly mount the mailbox). If you are mounting this mailbox to a metal or plastic bracket, you will need (4) #10 x 1 1/4" machine screws and nuts. If mounting to a wooden bracket, you will need (4) #10 x 2" self tapping wood screws.