Get whatever looks good and/or fits the lines of the plane.

I make templates out of paper; you can go to a website and save the picture of a spinner, then print it and resize it if needed. I hold it up in front of the plane and see how it looks, and it works pretty well. I also sometimes just cut them out of paper. Look at the dimensions of the spinner - if it is 3" spinner and 3 1/8" long, you can pretty much fudge the shape and make it look good enough to decide on the size.

Hope this helps.