According to the USPS, the minimum size of an envelope that can be mailed is 5 inches long by 3 1/2 inches high by 0.007 inches thick. The regulations also state that envelopes must be rectangular and not larger than 11 1/2 inches by 6 1/8 inches by 1/4 inches.
The maximum weight should not exceed 3.5 ounces. Letters that do not meet these requirements can be still mailed, but the USPS may assess what it describes as a nonmachinable surcharge, as these parcels must be hand-sorted. The USPS may also apply the surcharge to letters that have clasp closures, are lumpy or do not bend easily.