The current postage rate for a standard size postcard is 34 cents. This rate is as of January 2015 according to the United States Postal Service website.
The largest postcard size that still qualifies for standard postage is 4.25 inches by 6 inches. The smallest postcard for the standard rate is 3.5 inches by 5 inches. All postcards must have a paper thickness that measures between 0.007 inches and 0.016 inches. Postcards that are larger than the standard postcard size cost 49 cents each to mail. USPS offers a bulk mailing rate for large batches of postcards, which makes the price per postcard lower.