Print on card stock. If the size isn't satisfactory, try opening the file with Paintbrush, the Mac paint program. You can find the link to install Paintbrush here (http://paintbrush-mac.en.softonic.com/mac). If that doesn't work, or you don't have a Mac, try resizing the image yourself. The flat image should be almost exactly 10 inches in length (measuring from the top of the top box to the bottom of the bottom box).
Have fun! I made this, because I'm hoping I'll get the opportunity to throw a Harry Potter party for my future children! I'll upload templates for the Chocolate Frog cards too! If you have any questions or need further instructions, leave me a comment.