Mail is classified by the USPS according to its dimensions. Postcard size is defined as between 3 1/2 inches and 4 1/4 inches in height (measured vertically, perpendicular to address), must be between 5 and 6 inches in width (measured horizontally, parallel to address). The thickness of paper used in making a postcard size mail is also important. Postcard size mail must be made from paper that is between 0.007 inches and 0.016 inches in thickness.

Learn more at Postcard-Size.com.