In an ideal situation you want as little fletching as you can work with...but with selfbows and primitive arrows I find more to be better. I generally cut my feathers to about 7", peel off the front 3/4" of the feather and cut of the back 3/4"of the feather leaving the quill, front and back for tying. This gives me about 5 1/2" long feather. I like a high backed shield cut but also experiment with different burn patterns for better flight and aesthetically pleasing fletching.
I generally straight fletch with a slight offset.