Dressage riders regularly lunge horses, but usually starting on a 20 m circle, which is also the width of a typical dressage ring, which is usually either 20×40 meters (think of it as 2 20m squares put end to end) or 20x60m (3 20m squares put end to end). Young, green or horses being retrained are usually not ridden or lunged on anything less than a 20 meter circle until they have learned to walk-trot-canter and bend with suppleness and comfort. The required circle size gets increasingly small as the horse & rider move up through the levels, but lunging is still frequently done on the 20 meter circle, until warmed up, at which point the horse may be brount in to a 15m circle, then perhaps the 10 meter circle, with the handler standing on X, which is dead center of the ring. The idea is that the horse should follow the length of the lunge line in or out, while maintaining rhythm, balance, suppleness, etc. The smaller the circle, the better balance, rhythm, etc. the horse needs to maintain the circle without changing speed, balance etc.