The first thing to consider when creating your postcard is the U.S. Postal Service size requirements for its special postcard rate of 32 cents per piece. The postcard can be between 3.5 and 4.25 ...

But to qualify for mailing at the first-class mail postcard price, it must be at least 3-1/2 inches high by 5 inches long and 0.007 inch thick. It cannot be more than 4-1/4 inches high by 6 inches ...