When using a drop away, you can get away with just about any fletching choice you want.
You could stick with the 5" feathers, they will certainly do the job. Or you could try some of the new vanes on the market, such as blazers or quick spins. Its all up to you really.
My personal preferance is 4" left wing feathers. Fletched with a decent left helical setting. They stabilize regular sized fixed blade broadheads really well, are quiet,lightweight,and durable. Plus I just love feathers!
You have to look at the pro's and cons of feathers and vanes, and decide for yourself. There isnt going to be one clear winner here!
Hope that helps, have a good one---Matt