As of May 2014, the United States Postal Service charges 34 cents to send a typically sized postcard to a recipient in the United States. For larger postcards, the rate is 49 cents, and for recipients in other countries, the rate is $1.15.
For postcards up to 4.25 inches by 6 inches, the domestic rate is 34 cents. For postcards larger than that, the domestic rate is 49 cents, up to a maximum size of 6.125 inches by 11.5 inches. Larger postcards can be sent, but they are treated the same as letters; the postage will depend on the postcard's size and weight.
Postcards can be sent to any other country for $1.15 if the postcard is no larger than 4.25 inches by 6 inches. Larger postcards can be sent, but the postage will be the same as for letters. The exact rate will depend on the destination country, as well as the size and weight of the postcard.
The Postal Service also specifies a minimum size for postcards. For destinations in the United States, the minimum size is 3.5 inches by 5 inches; for other countries, the minimum is 3.5 inches by 5.5 inches. Postcards smaller than that cannot be mailed.