The U.S. Postal Service makes a distinction for postcards in First Class mail only. The minimum size is 3 1/2 x 5 inches, and the maximum size is 4 1/4 x 6 inches. Postcards may be printed on stock as thin as .007 inch. So called "double postcards", where half of the mail piece is a tear-off reply card, may also qualify to mail at postcard rates. Back
The USPS defines a "postcard" as a single piece of card stock measuring no less than 3.5" x 5.5" and no larger than 4.25" x 6". Cards of that size qualify to be mailed at postcard rates. However, if you mean a "card" that is going to be mailed, it can be much larger. A direct mail piece on card stock must conform to the same minimum size as a postcard (3.5" x 5.5"), but it may be as large as 6 1/8" x 11" and still mail at "letter" rate.