The minimum size postcard that can be mailed is 3.5" x 5". Here is some additional USPS information on the matter.

USPS requires a standard 1.25”x1.25” Indicia area and a 4”x2.5” addressing area. In most cases this is not a problem as the print area is much larger than the indicia and addressing area combined; however, when printing on 4”x6” postcards a significant portion of the card must be reserved for addressing and indicia.

If you want your artwork to qualify for Presorted Automation please make sure you follow the guidelines below. Templates can also be downloaded by clicking here to get a print-ready blank template for your artwork, that already meets USPS Automation standards. Click here to check our Direct Mail products https://www.4over4.com/printing/category/direct-mail-services