I can just say that you'll find that ruler handy many times when you read an instruction book or blue print and it says "use the 1/16-28 XYZ Dowel"

I think a heavy spinner nut is more for balancig (and good looks), than having any major impact on idling.
A longer prop though shoud improve idling since the engine gets loaded with more g-force and want to tick over again and again and again.... (Inertia? )