C500 describes maximum weight and size for Express Mail and standards for mailing outside parcels.
Each piece of Express Mail may not weigh more than 70 pounds. The combined length and girth of a piece (i.e., the length of its longest side plus the distance around its thickest part) may not exceed 108 inches. (Lower size or weight standards apply to Express Mail addressed to certain APOs and FPOs.)
Express Mail items not required to be presented in a pouch (outside parcels) must be large enough to hold the required mailing labels and indicia without bending or folding on a single optical plane.
DMM Issue 58 Updated 12-9-04