What is the rule of thumb on knowing what size spinner to use, is it the Plane or the engine size?
wattmeter

It's normally the plane. Most spinner sizes are dictated by the plane they are installed on. In planes with cowls the spinner will be sized to meet the lines of the cowl so that it's a smooth appearance from the cowl to the spinner. Engine and prop size aren't normally considered when determining the size of the spinner.

Ken

Thanks Ken
wattmeter

In one of the early Aeromodeller Annuals (1948+/-) it is said that the most efficient spinner is one that is 1/5 the prop diameter. ie 2 inch spinner for 10 inch prop. Sounds about right.

If there was any such thing as a rule of thumb it would be pointless because the the Rule writer did not know you.

All good conventional points by Ken and Jim but possibly the real bottom line is what appeals to your eye.

With the normal cautions that make sure the spinner sufficiently clears the prop hub and nut/nuts. Oversize is ok to a point however it can get excessive in which case it reduces the propellor blade swept area and this of course will reduce performance but as a practical matter it would only be in and extreme case.

For ordinary sport flying in what ever discipline you may choose the spinner is not that important to performance , Its a "That Looks About Right" thing TLAR[8D]

John