Letter Size

The USPS classifies a litter size mail a piece with the minimum height of 3.5" and minimum length of 5" but whose height is no greater than 6.125" and whose length is no greater than 11.5". The piece should also be at least .009" thick but not exceed .75" thick.

This is the most common size used, as most postcards, self-mailers, letters fall into this size.