While I agree with most of what has been said in here about aiming for hip width, I think that too many people have overlooked the importance of what Arcmeister stated. This is something that I argued time and time again with a PSIA DCL, and until I had an Examiner agree with me, the DCL would not back down, even though I was right the whole time.
Often times, as Arcmeister stated, hip width (people used to refer to shoulder width as well) is not how wide the hips appear to be, but where the femur connects with the hip. The way I describe width of stance with beginners is to try to stand with the feet the same distance apart they would be if they were walking. This stance it typically VERY close to true hip width, and it will be a stance that will be variable when needed.
When I work with people who's stance is forced wide, not allowing the skis to move simultaniously, I have them stand up (which brings the feet together), and bounce (standing slightly forward, using all the joints equally. Once they somewhat get this, I'll have them try to bounce through a couple turns, and most of the time they'll feel that the skis are working together.
When students stances are narrow, I will tell them, but not focus on it, instead, I will have them focus on getting the hip into the proper location for the turns and driving the skis, not leaning back and twisting excessively. Basically, by improving their turns, and correcting the way they "stack" the body, their feet often start to separate without them thinking about it.
All in all, my opinion when teaching, I will not teach "concrete" stance width, I instead focus on balance and stance (general) and focus on comfortable and effective width.