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.
Size & Weight Requirements. 1  Postcards Postcards must be rectangular to qualify for First-Class MailÂ® prices. 2  Oversized postcard will be charged as a letter or large envelope. 3  Size Requirements: Height: 3 1/2 min. / 4  4 1/4 max. 5  Length: 5 1/2 min.