Postcard Size
The USPS classifies a postcard as a card with the minimum dimensions of 3.5" x 5" and maximum dimensions of 6" x 9". This size is new as of August 29, 2021. The card should also be at least .007" thick but not exceed .016" thick.
The name is often deceiving as larger postcards are actually classified as letters. Postcard rates are first class rates and if you would like to send non-profit or standard mail they will be classified by the USPS as letters.