For a simple plastic spinner 2 or 2 1/2" will work. There's no advantage to going to the larger 2 1/2. When mounted, make sure the cutouts on the nose cone piece do not touch the prop when mounted. The Dubro ones for around $5 at the LHS are pretty cheap. Also, make sure the one you purchase includes an assortmoent of spacers so there's a good fit to the size of engine shaft you have.