Hi
there is no set length for hair rigs i have proved many times this depends on the lake you are fishing. I have tried 6 inch rigs and caught lots of big carp, then tried that menthod on another lake and never had a run.. Changed to a short rig 3 inches and started to catch... So you have to try a number of things and see what works on that lake... If the lake is very silty then use a popup works a treat