Applicability of the New Rules. The new cargo securement rules apply to the same types of vehicles and cargo as the old rules, covering all cargo-carrying commercial motor vehicles (as defined in 49 CFR 390.5) operated in interstate commerce.
Most commercially-produced postcards measure approximately 4.21 inches by 5.47 inches. Less commonly, smaller postcards are typically 3.5 inches by 5.5 inches, close to the minimum size the USPS allows.
Use these rulers and guide boxes to measure letters and large envelopes. Once a piece of mail exceeds the maximum length, height, or thickness of one shape, it automatically gets classified as the next largest shape. Minimum size for Postcards and Letters. 5 long x 3-1/2 high.