What size is over sized for U.S. Mail?
There are two sizes for business mail, card size (postcards) and letter size. The card size has a minimum of 3.5″ x 5″ and a maximum of 4.25″ x 6″ and costs less to mail than the letter size. The letter size has a maximum size of 11 1/2″ x 6 1/8″ and costs $0.39 per piece first class. The post office charges a surcharge for square pieces of mail. To determine if your piece is within the allowable standard calculate the aspect ratio. Divide the length by height, if the number is less than 1.3 or greater than 2.5, there will be a surcharge.
