A letter size piece must meet specific USPS parameters:
Not more than 11-1/2 inches long, or more than 6-1/8 inches high, or more than 1/4-inch thick.
Not more than 3.5 ounces (First-Class Mail letter-size pieces over 3.5 ounces pay flat-size prices).
Rectangular, with four square corners and parallel opposite sides, and an aspect ratio (length divided by height) of more than 1.3 and less than 2.5.
Minimum: 3-1/2 inches high, 5 inches long, and either 0.007 inch thick if not more than 4-1/4 inches high and 6 inches long; or 0.009 inch thick if more than 4-1/4 inches high or 6 inches long, or both.
Maximum for cards at card rates: 4-1/4 inches high, 6 inches long, and 0.016 inch thick.
Maximum for folded self mailers and booklets (a folded self-mailer is formed of two or more panels that are created when one or more unbound sheets of paper are folded together and sealed to make a letter-size mailpiece): 6 inches high, 10-1/2 inches long, 1/4 inch thick. Maximum weight 3 ounces