Just to be different, I printed them four-up on post-card stock and wrote on the back, then stuck them in envelopes to mail. Instead of a folded card, it's simply a post card. That seems to be a trend with photo Christmas cards these days, so I'm stealing the idea both for simplicity and economy.
(You could, of course, set the image up to print on foldable card stock. Final size is 4.25" wide by 5.5" deep, and fits in an invitation-sized envelope.)
Clicking this link will take you to the download vault, where you can snag a Photoshop version of the card for yourself. You're welcome!