So, the first thing I needed was a chalkboard background, which it turns out I had to make because if you want to print something in large format, no such background exists out on the internet. I found some great instructions here--but I've uploaded my version so you can just use mine if you want. It is HUGE, 20x30 and at 300 dpi, so you can print it (I think it's a 12 MB file).
I did the Joy print in both 20x30 and 8x10. I suppose you could try it at other sizes too--should have plenty of pixels to print whatever you want. I'm doing the giant size from Costco for $8.99 and will put it up in the mirror frame already on the mantle.
Blank Chalkboard background 20x30
8x10 Joy to the World
20x30 Joy to the World