Learning how to lunge is an art (ie whip should always point at croup and lunging line hand towards the head). So, the whip's lash has to be long enough to brush an appropriate part of its anatomy (usually the lower part of the bit/hindleg) with the end of the lash. That means either a long whip (which can be heavy) or a longer lash (usually leather). A telescoping whip is a good idea, but they are expensive AND the handler has to be VERY careful never to drag it on the ground. In any case usually 6-8' whip with 12' lash.