For first class, standard and international letter class mail weighing one ounce or less, the following are considered STANDARD SIZE LIMITS:
6 1/8 inches in height
11 1/2 inches in length
1/4 inch thickness
Aspect ratio (length divided by height)
between 1 to 1.3 and 1 to 1.2.5 inclusive
NOTE: There are also mail dimensions which are non-mailable by USPS and which will be returned to originator. See Enclosures.