For a .40, I've seen spinners ranging from 2" to 2 1/2". My two .40 size planes have 2 1/4" and 2 1/2". If your SPAD is on the skinny side (like most spads are), I suggest going with a 2" spinner.

Oh...and get one with a metal backplate so that you can really crank down the prop nut and hold that baby on (plastic backed ones will tend to spin off with a starter and can't take as much tightening).

Jim