Here are some of my tips:
*If ever there's no indicated size or feature of the Christmas card, you can make it in a SCROLL form(the one used in the ancient times) like the one here in the picture.... http://www.compassionatexpression.com/Fantasy%20Scroll.jpg
*Add some decorations like artificial red flowers and green leaves (because red and green are the motif colors of Christmas) such as poinsettia. Here's the image:
http://www.gefanuc.com/pressroom/literature/20041223_poinsettia.jpg
You may also add some eye catching letter fonts and glitters...
*After you wrote your heartwarming words in the letter, you may close and wrap it with an enclosed ribbon.
*If you want, you can put the scroll (your letter) inside a wine bottle like this:
http://www.magic.org/store/images/neilsen_vanishing_bottle.jpg
*Before you put inside the scroll inside the wine bottle, you can put some confetti inside the bottle and some other crafts you like.
I hope this will work out... If you want some more suggestions and tips, feel free to send me an email...