A diameter of 1 1/4" works fine. Check the face of your prop. Then make it as long as you need it. As you can see from my picture, it can run just fine pretty close to the engine if you don't need a longer extension for cowl clearance. Drill and tap to fit the crankshaft, and drill and tap the other end to fit the prop bolt or stud you intend to use. If you have a mill, milling a couple of flats to fit a wrench is nice. This is all assuming you are going to run the magneto. If not, you have to figure a way to adapt to the crankshaft taper. What I do is use a hole saw to drill out the center of the magneto, and I use that as my adaptation to the taper.

Good luck,
AV8TOR