The USPS actually has very strict criteria for what qualifies as a postcard, which qualifies for it's own special, first-class mail rate. From USPS.com:
• At least 3-1/2 inches high x 5 inches long x 0.007 inch thick
• No more than 4-1/4 inches high x 6 inches long x 0.016 inches thick
Anything larger than the above dimensions is classified as a "letter" and is charged as "flats"(ie large envelopes).