4" helical should be sufficient for most archers
Helical vanes are a bit hard to make. Most shops around here anyway won't even do them. I have made them in the past using quick set glue, but because of the memory of the vane, you really have to let the glue set up before removing them from the jig.
But anyway, when I shoot fixed blade broadheads, if I am using a vane (which isn't often anymore), I use a 5" offset setup. I prefer feathers now however.
If you are just shooting field points, you can definetely get away with using less then a 5" vane.