The USPS classifies a postcard as a card with the minimum dimensions of 3.5" x 5" and maximum dimensions of 4.25" x 6". The card should also be at least .007" thick but not exceed .016" thick.
Postcards are relatively small and can only be mailed at First Class rates. This size is generally used for reply cards or informational mailings that need to be mailed at first class rates. The name is often deceiving as larger postcards more commonly used in direct mail marketing are actually classified as letters.