As of June 2014, the United States Postal Service states that the cost to send a standard sized postcard is 34 cents. At this rate, the postcard can be 6 inches long, 4 1/4 inches high and 0.16 inches thick.
The USPS also includes that a large postcard is 49 cents to mail. The maximum size for this postcard is 11 1/2 inches long, 6 1/8 inches high and 1/4 inch thick. This is the same rate to mail a First Class 1 ounce letter domestically.
The rates for postage increased as of January 2014. Metered mail is 1 cent less than the retail rate for a standard postage stamp.