True, the plane dictates the spinner size. On a trainer most folks just run a spinner nut. Just a nut that's cone shapped like a small spinner. It's easier to remove and install during prop changes and it allows you to use a starter. I had a OS .91 four stroke engine in one plane with a 3 inch spinner on it so it looked good/fit the pane. I have another OS .91 four stroke in an old Kaos that I built a cowl for, it has a 1 1/4 inch spinner on it. That's about as small as will fit on that engine and still have enough mass for a prop to fit behind, it's a tight fit!! Anything bigger and the plane would look stupid, friends do let friends fly stupid looking planes!!![8D]