The U.S. Postal Service divides mail into standard and non-standard sizes. An envelope is classified as non-standard if it exceeds 6 1/8 inches in height, 11 1/2 inches in length, 1/4 inch in thickness or if its length divided by height is less than 1.3 or more than 2.5.
While postage is generally based on a package's weight, lightweight items that exceed certain sizes receive a surcharge due to the amount of space they take up during transportation. The USPS uses three methods of applying extra charges: dimensional weight, balloon weight and oversize weight. Which charges are applied depends on the mail class, weight, size and distance. The dimensional weight is applied to Priority Mail items larger than one cubic foot. The balloon weight is applied to Priority Mail and Parcel Select items whose length plus girth are between 84 and 108 inches. The oversize weight applies to First Class, Standard and International Letter Class Mail weighing 1 ounce or less that exceed any one of the following: 6 1/8 inches in height, 11 1/2 inches in length, 1/4 inch in thickness or if its length divided by height is less than 1.3 or more than 2.5.
The largest domestic package size that can be shipped via the USPS is 130 inches (length plus distance around the largest point) and must be shipped Parcel Select. International package size limitations vary by country.