Charts for envelope sizes are available online at Paper-Papers.com and on the U.S. Postal Service website. If the size of a package is an issue for mailing purposes, a mailer should determine exactly which size and type of envelope is required and the weight of the completed package.
Paper manufacturers make several types and sizes of envelopes. The standard and most-used envelope size is the #10 letter envelope, while the envelope style most used for business and personal use is the "commercial flap." Other common sizes include booklet, catalog, square and announcement, among others.
The U.S. Postal Service lists three specific guidelines for an envelope to be mailed at the standard letter rate. It must be rectangular and can be no more than 6 1/8 inches high and 11 1/2 inches long and no more than 1/4-inch thick.
When mailing envelopes larger than the standard #10 envelope, additional postage may be required. Large catalog-size envelopes aren't "machine friendly," which means they can't be sent through a postal sorting machine. They also can't be sent through a laser printer.
Postage rates are based on various factors in addition to the envelope size. The shape, content and travel distance are among the criteria.