I know the prop im using is quite large but i have the engine in a bud nosen trainer and the fuse is about 8 inches wide so i have to have a prop large enough to get around the sides.
An extra half inch on each side isn't getting you much. Actually you would be suprised how effective a prop is when the back wash is hitting a surface behind the prop. Yes it hurts its performanc, but probably not as much as you would think.