scope size
If you shoot field or 3D, you need a large scope so more light will come in (The woods are often dark) and secondly so you can see more of the target face in Field.
If you shoot F.I.T.A. style and your draw is less then #50, then a smaller scope is what you need, so when you shoot 70-90 meters your vanes will not hit the bottom of the scope.
Hope this helps.
Mike.