I checked my Dresden plate quilt (a gift from a friend) and it has 12.5" plates and is on a 15" finished background. It has a 3" finished sash. I have some plate blocks I bought at an estate sale and they are 8.25" on 12" unfinished background. The sashing for that one is cut at 3" unfinished.
I'm with klswift, just try it on different sizes and see what you like. Dummy up the sashing, too and see how wide you think that should be. Dresden Plate is so pretty, you can't possibly go wrong.
bkay