What determines the size of your spinner plane, engine or prop

Plane

To some degree, maybe both. Most .40 sized engines take a 2 to 2 1/2" spinner and are generally running a 10 to 12 inch prop.. On bigger engines, a bigger spinner may be required due to the size prop you are using or because an electric starter may slip on a smaller spinner, the bigger the spinner, the more grip the starter can get on it. My Magnum .46 has a 12x4 apc prop that wouldn't fit in my 2" nylon spinner, so I'm using a 2 1/2" aluminum spinner on it. Just my experience, some one else may have a more exact answer. Hope this helps.

Generally a plane with a cowled engine wants a spinner that matches with the cowl for a nice, smooth transition. If a plane doesn't have a cowl, I still try to get a spinner that would flow smoothly with the cowl that should or wouldbe there.

exactly

It depends all on what you want. I used a larger spinner on both my extra and my mach 2 than it called for. It flowed with the cowel better on the extra and the 3.5 incher on the mach 2 made it a faster plane because it got the air out around the square nose.