3.5 x 5.5 inches photo postcard.  Please use postcard shipping method.
 
 
As per collectors weekly --

Real-photo postcards (sometimes called RPPCs) are snapshot or photos printed on a postcard back...Though the first documented photo postcard was mailed in 1899, the style wasn’t firmly established until Eastman Kodak began selling Velox photo paper with a pre-printed postcard back in 1902.