Bill worked out the dimensions for me. My block is 9"x12" so the image has to be approx. 6.5"x11.5". I'll make it a bit wider, but it'll be detail that can be easily cropped out of the card image ~ more wall). Card image is generally extra long to width ( ≈ 4.5"x7").
I'm also thinking of adding something in the foreground ~ say a doggy dish with bone-shaped biscuits for Santa (like a plate of cookies).
I like the plate of cookies. Gives it foreground interest.
Green trim on the cookie plate balances the green collar. Anchors the image.