Domestic Classes of Mail
The cost of mailing varies with each classification.
First-class, Standard Class and International Letter Class Mail weighing one ounce or less is nonstandard* if it exceeds any of the following size standard limits:
11 1/2" in length, 6 1/8" in height, 1/4" in thickness
Its aspect ratio (length divided by height) does not fall between 1 to 1.3 and 1 to 2.5 inclusive.
* There is an additional postage charge on each piece of mail that is found to be nonstandard.
In addition, pieces which are less than the following dimensions are non-mailable in the U.S. Postal Service and will be returned to you:
3 1/2" in height, 5" in length or .007" in thickness (thickness of a postcard)