ORIGINAL: RCKen
The spinner size is usually determined by the plane that it is installed on. In the case of planes with cowls the spinner will be size so that a clean smooth line is created between the shape of the spinner and the cowl. For planes without an enclosed cowl it's not as critical for the size of the spinner.
Ken
My trainer has no cowl. I'm using an OS40LA. 10 inch prop. I plan on using a starter. Any suggestions on spinner size?