The minimum size envelope that can be sent through the United States Postal Service is 3.5 inches high, 5 inches long and 0.007 inches thick. It has to be rectangular in shape and weigh 3.5 ounces or less.
Odd-shaped envelopes or those whose contents are lumpy, rigid or more than .25 inches thick are accepted but incur a nonmachinable surcharge. Putting the address parallel to the short end of the envelope or using an envelope with a clasp can also trigger the surcharge. Envelopes whose length divided by height is less than 1.3 or more than 2.5 are considered odd shaped. If a smaller or unusual shape is preferred, a package can be used, as these have no minimum size limitations.