If you can reach a fully extended position halfway back, with the clubshaft parallel to the target line, you've virtually guaranteed yourself a wide arc. From there, lift your arms to the top of the swing, as the wrists naturally hinge and the shoulders continue to turn. Creating space between your hands and body on the backswing gives you room to swing your arms on the downswing. It's not just as simple as a big turn, though. A big turn is nice, assuming the weight is firmly planted on the inside of the right foot on the backswing, but it doesn't guarantee power.
Power comes from torque, the twisting motion of the torso.
To create torque and whip the clubhead through impact, the shoulders and hips must work independently. The longer the shoulders stay wound, the more separation there is between the hips and shoulders. Finally, there is so much rotary tension that the shoulders are forced to unwind, bringing the arms and club with them in a powerful whipping motion. Ask instructors and experts in the Instruction Forum .