It depends on the geometry of your polar mount if it is even able to go that far without flopping over. My 6 foot dish had an 18" actuator, and that worked just fine. My 10 foot dish has a 24" actuator, and that works fine for that one... I would suspect an 18" would work for you as well.
It's easy to measure it with a tape measure before-hand by manually moving your dish, and then check to see what the maximum extension is on a particular actuator.
decide what side to put the arm on. determine the side that will have the greatest arc, east or west of your location. Almost everyone will have either more visible satellites on one side or the other. ie me I live at 113w and can view down to 58w and upto 137w. so I have more arc on the east, therfore the motor goes on the east side.
ruffly aim the dish at your true south, then move the dish roughly to your farthest satellite so the acutator would be extended the most it would be on the arc. now measure how long of an actuator you need.