Good afternoon from Classic Swing Golf School in Myrtle Beach! What exactly is Stance Width? Read below!
Stance Width
Based on the acronym G.A.P.B.S. used in our school to identify golf’s five fundamentals, we place a handle on Grip, Aim, Posture, and Ball Position and now find ourselves considering the topic of Stance Width.
I am a believer in one ball position and not moving the ball all over your stance based on which club you have in your hand. I will suggest varying the stance width based on a short, mid or long club being used. To keep it simple, categorize all short irons as the 8 iron through all the wedges; the long to mid irons as the 2 hybrid through the 7 iron; and the long clubs as the driver through your longest fairway metal. 
Picture 1 illustrates a simple method to initially address the ball with your feet together and the ball in the middle of your feet. It is always the movement of the lead heel (the left for the right-handed hitter) that controls ball position. My hot spot is to situate the ball 4 inches in back of the lead heel for all clubs. However, the key for this month is the movement of the trailing heel (right foot).
With the short irons, the right heel moves 8 inches from its original position to create a 12 inch total space between the heels (See Picture 2 at left).
For the long to mid irons, the trailing heel moves 10 inches, for a total of 14 inches between heels.
With the long clubs there should be a 12 inch movement of the trailing heel thereby supporting our widest stance, 16 inches, seen at the right.