If you learn to bare-shaft tune your arrows, a minimum amount of fletching is all you need to keep the arrows on course. 3" fletching would be fine for tuned arrows used with field points.
Since you're not sure of what size arrow to use, I assume you haven't done any arrow tuning, so you will probably need bigger fletching to straighten the arrow out. If you intend to shoot broadheads, you need bigger fletching, tuned or not. For someone who has not tuned the arrows to the bow, or for someone who wants to shoot broadheads, I would recommend 5" fletching. Some people want to shoot the same arrows for practice as they do hunting, and will use 5" fletching for everything. The only disadvantage to that is that the arrow velocity will drop off considerably after about 30 yards.