For mail sent through the U.S. Postal Service, postcards must be at least 3.5 inches high by 5 inches long by 0.007 inch thick and no bigger than 4.5 inches high by 6 inches long by 0.016 inches thick. The standard-size British postcard is 3.5 by 5.5 inches.
Court cards were the standard-size postcard in Britain in the late 1890s. They measured 4.5 inches by 3.5 inches. Intermediate British postcard size is 5.1 inches by 3.1 inches. Midget postcards are 3.54 inches by 2.76 inches. Postcards are intended for mailing without an envelope and typically feature a picture or infographic on one side and a space for the address, postage and message on the other side.