define usps large envelope
USPS Large Envelopes. What are Large Envelopes. The term âlarge envelopesâ (also known as flat mail) refers to First Class Mail sent in envelopes that exceed either 6ââ in height, 11Â½â in width, or Â¼â in thickness. Large envelopes are often comprised of items such as marketing newsletters, product catalogues and legal correspondence.