The most economical postcard size is 4” x 6” because it qualifies for first-class delivery times and the standard mailing rate with the United States Postal Service. According to USPS, your postcard needs to meet the following requirements to be mailed first-class:

Rectangular
At least 3-1/2 inches high x 5 inches long x 0.007 inches thick
No more than 4-1/4 inches high x 6 inches long x 0.016 inches thick