Printer specifications are real and accurate and meaningful, but are not to be confused with image resolution. Printer ink dots and image pixels are simply very different things, and one color image pixel requires many printer ink dots. This is why we need a 1200 dpi printer (ink dots) to print an image at 250 dpi (pixels).
Q: What are the standard postcard sizes you offer? A: For flat standard postcards, available sizes are as follows: 4 x 6, 4.25 x 6, 5 x 7, 5.5 x 8.5, 6 x 9, and 6 x 11. Folded postcards have 2 kinds: half-folds and tri-folds.
Depending on the mail service you are using, the cost and requirements for a postcard vary. The USPS considers a postcard to be any single rectangular piece of paper that is at least 3 1/2 inches high by 5 inches long by 0.007 inch thick' and no more than 4-1/4 inches high by 6 inches long by 0.016 inches thick.. The post office considers all papers outside of these dimensions to be letters.