Both must be 256x256 Pixel Size. Also, remember to trans. If you don't, your decal/face will end up looking really forgeted up. When packaging the decals/faces, include a "Thumbs" File but leave it empty. The new decal system automatically creates icons for you in the Thumbs folder.
EDIT: But you must include and create an icon for Prints/letters, or else they just plain won't show up.
