You probably have a picture in your mind of what a "letter" is. Possibly you’re using a standard No. 10 envelope for your letter, so that’s easy to picture. However, if you decide to create a mailpiece with its own special envelope or if you are designing a piece that will be folded to letter-size, be sure to keep in mind the following size requirements for letters:

To be eligible for mailing at the postage rates for letters, a piece must be:

• Rectangular
• At least 3-1/2 inches high x 5 inches long x 0.007 inch thick.
• No more than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick.

First-Class Mail letters that has one or more nonmachinable characteristics[1] will pay a nonmachinable surcharge. Customers can be unpleasantly surprised that they must pay extra postage when, for example, they mail a square greeting card. The Postal Service charges extra postage because mailpieces that are too stiff, too square, or unusually shaped often jam postal equipment and are difficult to process. This costs the Postal Service time and money—and may ruin your mailpiece.

Many mailers fold sheets of paper to form letter-size pieces or newsletters. Folded pieces can save time and money because you’re not paying for or stuffing envelopes. We recommend that you use wafer seals or tabs to close the open sides of folded mailpieces.

Here’s some good advice on tabbing: Put the fold on the bottom (the side below the address) and secure the top flap with a piece of tape or a round seal.

More about Folding and Tabbing a Mailpiece[2]

There are special size requirements for mailing letters at automation rates. Contact your local business mail entry unit[3] for more information.

TIPS
-0.007 inches? How do I measure that? As a guide, an index card is thick enough. If in doubt, contact a mailpiece design analyst (MDA[4]) at the Post Office near you. MDAs have tools for precisely measuring thickness and can tell you if your mailpiece is thick enough.
-What is high? What is long? Length is the side parallel to the address. Height is the side that is perpendicular to the length.

Sizes for cards[5]

Sizes for flats[6]

Sizes for parcels[7]