The U.S. Postal Service sells envelopes in sizes ranging from 5-by-10 inches all the way up to 11 5/8-by-15 1/8 inches. Most of their envelopes are dedicated to Priority Mail for items needing urgent delivery. The Postal Service also sells shipping boxes for larger items.
The U.S. Postal Service sells standard flat rate Priority Mail envelopes in sizes including 9 1/2-by-12 1/2 inches, 9 1/2-by-15 inches, 6-by-10 inches, 10-by-7 inches and 5-by-10 inches. Priority Mail Express envelopes come in sizes including 9 1/2-by-12 1/2 inches, 9 1/2-by-15 inches and 5 7/8-by-10 inches. Padded Priority Mail and Priority Mail Express envelopes are available at 9 1/2 by 12 inches, and waterproof Tyvek Priority Mail envelopes are available at 11 5/8-by-15 1/8 inches.
For international mail, the U.S. Postal Service sells Global Express envelopes at 9-1/2 by-12 1/2 inches and 11 5/8-by-15 1/8 inches.
For shipping larger items, the U.S. Postal Service sells Priority Mail flat rate boxes in sizes including 5 3/8-by-8 5/8-by-1 5/8 inches, 11 7/8-by-3 3/8-by-13 5/8 inches, 23 11/16-by-11 3/4-by-3 inches and 11-by-8 1/2-by-5 1/2 inches. Specialty boxes are available for shipping shoes, DVDs and board games. In addition, mailing tubes are available for shipping rolled-up documents such as posters.