I usually us 5 or 5 1/2" shield cut feathers on cedar arrows. I don't know if it's a NEED thing. They just seem to look right on woodies. Your arrows will also be somewhat more forgiving and stable. There may be some loss of speed, but I've never really noticed the difference.
I use 5" feathers on my carbons with those big old Zwickey Deltas up front. Again, 4" feathers would probably do the trick, but I like to make sure I stabilize the arrow with those wide broadheads up front, especially if there is any wind.