The correct answer is....it depends on what your doing and behind what sort of boat ?
A standard length is 75 foot,
Behind a Correct Craft Nautique or Mastercraft 190 etc then that will be too long for the slalom as the boats wake is set at 36mph to be ideal, if your trick skiing then id shorten it down to 50 and the boat at 26 will give you a ride on a table top
So like i say, its boat and speed dependant and also dependant on what discipline your skiing