Sizes for Parcels | Postal Explorer - USPS At least 3 inches high x 6 inches long x 1/4 inch thick. Except for USPS Retail Ground and Parcel Select, no mailpiece may measure more than 108 inches in length and girth combined. Length is the measurement of the longest dimension and girth is the distance around the thickest part (perpendicular to the length). Maximum weight is 70 pounds.