The United States Postal Service (USPS) has specific rules when it comes to envelope sizes when you're sending packages and mailings. The measurements are important because the post office's mailing machines can only accommodate certain sized envelopes. An envelope mailing is almost always eligible for First-Class mail service. If you don't observe the post office's regulations regarding size, a USPS representative could reject the mailing and return to sender.
Letter Size Envelopes
The USPS requires your letter-sized envelopes to be a minimum of 3.5 inches tall by 5 inches wide and a maximum of 6.125 inches tall by 11.5 inches wide. The post office also has strict requirements regarding the thickness of the envelope---between .007 inches to .25 inches thick. If the envelope is too thick or thin it could cause problems with the post office's machines.
If you need to send something larger than a standard-sized envelope via the USPS, there are regulations regarding large envelopes, as well. A large envelope is any envelope that is bigger than the standard letter size as described previously (6.125 by 11.5 inches by .25 inches thick). Your envelope cannot be bigger than 12 inches tall by 15 inches wide. The thickness requirement goes up to .75 inches for a large sized envelope. Any piece of mail that is larger than this size is not considered an envelope---it is parcel, which is priced differently from First-Class mail. Keep in mind that a piece of mail is only eligible for First Class if it is under 369gr, otherwise you have to use a more expensive service such as Priority Mail.
If you have an envelope that exceeds the depth requirements of a large envelope (.75 inches), but it's still under 369gr and meets the size restrictions (up to 12 inches tall by 15 inches wide) it's considered a thick envelope or package. You can still send this type of mailing via First-Class, but be prepared to pay a higher fee. Any thick envelope over 369gr in weight is quoted for Priority Mail, Media Mail, Parcel, or another service other than First-Class.