Who says size matters? Our U.S. Postal Service does, for one. If you are thinking about building your own mailbox or considering a purchase of something more eye catching than the basic bread-loaf-shaped official mailbox, you may want to keep a few things---size among them---in mind. You cannot get around some things, and the U.S. postal regulations concerning mailbox sizes are--like death and taxes--among those. Next time you feel like choosing from the myriad cute designs and architectural styles available, consider whether the insides of your cow, fish or masonry mailbox conforms to regulations.
The United States Postal Service issues specific guidelines concerning the mail. Instead of the "tomato cans, cigar boxes, drainage pipes upended, soap boxes and even sections of discarded stovepipes ..." complained about by Charles Smith, U.S. Postmaster in 1889, and quoted in a recent TIMES article, the USPS now requires boxes to be "a minimum of 19 inches long by 6½ inches wide by 8½ inches high; maximum, 23½ inches by 11½ inches by 13½ inches," so don't think you can just stick a barrel out there to cover all your bases. However, all creative hope is not lost. With the proviso that it must also be "rust free and 'neat'" the post office only regulates what goes into the box, not what it looks like outside, so thinking "outside the box" is definitely appropriate here.
You can even build your own box from scratch provided you follow the rules stated here, from the USPS manual concerning "customer mail receptacles."
"3.2.2 Custom-Built Mailbox
The local postmaster may approve a curbside mailbox constructed by a customer who, for aesthetic or other reasons, does not want to use an approved manufactured box. The custom-built box must generally meet the same standards as approved manufactured boxes for flag, size, strength and quality of construction."
If you are thinking now would be the time to go ahead with that glitter-caked pink dragon you envision to match your castle, just be sure its tail (or faux fiery breath) extends no farther than it should for the comfortable insertion of mail by your local postal worker. According to the post office, the mailbox should be installed "with the bottom of the box at a vertical height of between 41 to 45 inches from the road surface..." and "set back 6 to 8 inches from the front face of the curb or road edge to the mailbox door."
The post office does not regulate shape or material of posts, but you should always consult local and state ordinances before you set yours in concrete.