size of postcards for us post office
The United States Postal Service defines a postcard as: rectangular, at least 3 1â2 inches (88.9 mm) high Ã 5 inches (127 mm) long Ã 0.007 inches (0.178 mm) thick and no more than 4 1â4 inches (108 mm) high Ã 6 inches (152.4 mm) long Ã 0.016 inches (0.406 mm) thick.