To rotate a system, torque is required. Torque is given by the formula
Torque = Force * Perpendicular Distance
Here, the perpendicular distance is the length of the string.
As torque is directly proportional to the perpendicular distance, it becomes evident that longer the string, more torque is required to rotate it. Hence we can conclude that it is easier to revolve a stone by attaching it to a shorter string.