envelopes
The size of envelopes used in mailing correspondence should properly accommodate the contents. For use in the United States Postal System there are basically two categories of envelopes: letter size and “flats,” as referred to by the USPS.
Letter Size Envelope Must be rectangular in shape for automated processing by the United States Postal Service. The following dimensions must be met for automated processing: height between 3 ½” and 6 1/8”; length between 5” and 11 ½”; thickness between .007” and ¼”.
Flats Size Envelope Large envelopes that exceed any one of the maximum dimensions of a letter are called “flats.” The maximum size of a large envelope is 12” high, by 15” long by ¾” thick. Mail pieces that exceed the maximum dimensions for a large envelope are charged parcel rates.