Larger feathers will help to alleviate underlying tuning problems, or help stabilize large heads quicker--as stated above.
If you bareshaft tune your bow, and get it set up properly, then you can go to smaller fletching.
I'm shooting about 4.5" medium-profile parabolics with large snuffers up front, and it's worked out pretty well.
I also picked up some store-bought 4" 'ers for another bow, because they didn't have shafts only, and see no difference between those and my other 5" 'ers that I had on hand.
You just have to experiment, and ensure you are tuned properly in the first place.
Take Care,
Marc