Postal Standards - The Graphic Design: Reference and Specification - page 119

subject to surcharge
4¼“ × 6” (10.8 × 15.24mm) Maximum Postcard size
3 1 ⁄ 2 ” × 5” (8.89 × 12.7 mm)
Minimum Mailable size
5” (12.7 cm)
6” (15.24 cm)
11 1 ⁄ 2 ” (29.21 cm)
Maximum standard Length