postal service website, a postcard must be a minimum of 3 ½” x 5” and maximum of 4 ¼” x 6”. Print an appropriately sized image on one side of heavy cardstock then cut it out. On the back of the card, be sure to leave the right half for the address and postage. Write your message on the left half of the back of the card. Want to score points with your romantic interest? Print a photo of the two of you, write a cutesy message and mail it. Even if you live together, your paramour will be tickled to get a personalized love note in the mail.