I use a 54-58" paddle with my shredder, personal preference. A lot depends on the type of river your running. Cheoah, Ocoee vs Upper/Lower Gauley. Rocky, shallow rivers are not conducive to a long paddle. You will bang your blade all day long and very likely will have it pulled out of your hand by the gremlins. A deeper current river, Gauley, would allow a longer paddle shaft.
I prefer to use customer sticks if possible. Lightweight and you can make quick, short strokes. Easy on the shoulders. Try different lengths and see what works best for you for a particular river. Dan