The maximum weight for a First-Class letter envelope, which is 3.5 ounces, according to the United States Post Office. There are also size and proportion requirements to consider when sending mail First-Class. Letter envelopes must be rectangular, and they must not exceed 11.5 inches in length or 6.125 inches in height.
Width restrictions for First-Class envelopes allow up to 0.25 inch, but cannot be less than 0.007 inch thick. Large envelopes shipping First-Class must also be rectangular in shape, and they must not exceed 15 inches in length, 12 inches in height and 0.75 inch in thickness. The maximum weight for First-Class large envelopes is 13 ounces.