Spinner sizes aren't dictated by the propeller size. They are usually (although not always) by the fuselage of the plane they are on. To be honest here, it's rare that you will get a spinner prop combination that doesn't require some bit of trimming of the spinner to get the prop to fit properly. When trimming a spinner care needs to be taken to trim the same amount of material from both sides of the spinner in order to keep it in balance. An easy way to do this is to mark the spinner with the area you want to remove. The way you can make sure that you remove the same amount of material from both sides as you trim it.

Ken