What are the most common sizes for postcards? 4” x 6”, 5” x 7”, 5.5” x 8.5”, 6” x 9”, and 6” x 11” are most common sizes for postcards. It is important to consult with us in advance as the postage cost may vary substantially due to the size and layout of your postcard. 2021-07-06T08:32:19-04:00 Share This Story, Choose Your Platform! FacebookTwitterLinkedInEmail