Vanes serve two purposes: the first is to correct a poor release and the second to stabilize the arrow by rotation. If you are shooting a mechanical release the smaller vanes will reduce weight and increase speed, but there is little use in a staight vane on the arrow. With no revolution in flight your arrow will be inconsistent with a broadhead attached. A small degree of offset will be enough for revolution and a true arrow flight. If you are shooting fingers I would suggest feathers over vanes no smaller than 4" to correct for a bad release.