In Straightness Training we use the whip as a subtle pointing device.
The whip should be as light as possible and you should choose a good whip that is easy to handle and fits the size of your horse.
So with very small horses you'll use a size 1.10 m / 3 ft in groundwork, work in hand and riding. With very large horses you might use 1.30 or 1.40 m / 4.5 ft. Most common is 1.20 / 4 ft.
With the whip the trainer can extend his arm and energy.
The whip should always be used in an animal friendly way and the horse should never be afraid of the whip. It's never the whip that is uncomfortable for the horse, it's always the human behind the whip that makes a whip uncomfortable.
Check out this article: It's Always the Human behind the Tools >>