Either 20 or 22 would work. Since it's a single dresden and I am guessing a table topper, the background size depends on how big you want it. If you cut it bigger you can always trim it down in case you don't quite get it centered.

Don't let it scare you. Dresdens are pretty easy to put together. I never bothered trimming out my fabric from the back. Just remember to use a lo of pins to hold it in place while sewing it together. You can also use spray basting if you prefer that.