How do you determine what size spinner to put on a plane?

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

Looks, cowl and stuff like that generally. You obviously don't want one so small that you can't cut it enough to clear the prop or one so big it covers too much of the prop blade of course. But I do not believe there is a set in stone rule. Maybe there is but I've never seen anything anywhere.

My trainer has no cowl. I'm using an OS40LA. 10 inch prop. I plan on using a starter. Any suggestions on spinner size?

2"

Thanks.

Or just a spinner nut.

dumb question on spinner nuts, seen pictures with planes using them, some have the prop washer and some don't, does it matter?

The nuts themselves are the same size or very close to the washer so in most cases no. On some of the older engines the crank shaft threads were cut shorter and the washer was needed to make up the space.
I just posted this photo of three different typre of nuts. The two nut shaped are Higgly's and the brass one is used for planes that are a little tail heavy instead of adding lead.
These are very cheap compared to a good spinner and don't break like the cheap plastic spinners.