Depends on what you want to do and why.

If it's a scale plane, then the spinner is set by the airplane.

For open / non-scale planes, we usually use spinner nuts rather than plastic spinners since you can change a prop much much faster with just the spinner nut on there. Dubro, Higgly, and others make some good ones.

Basically spinner size isn't really determined by the engine at all, it's deterimened by the plane and what you want to do with it.