For what it's worth, I'd say my typical "home-base" stance puts 4"-6" of snow between my skis. Sure, I'll make them as wide as I need to, whenever I need to, situationally, but a basic stance should be natural and uncontrived.
I would beware of any
general instruction to adopt a stance of any particular width--narrow, medium, wide, shoulder-width, hip-width, six inches, etc. Stance is a personal thing, for one thing, and the only real measure of "right" is function. How does it work? Stance width affects balance, edge control, pressure control, and rotary (pivoting) movements.
Effective technique today relies on an "open" stance--which should not be confused (although it often is) with a wide
stance. An open stance simply allows the two feet and legs to work independently, without interference from each other, and allows each to support movements of the other. Feet do not need to be very far apart for a functional open stance!
I like to say that your stance in skiing should be exactly as wide as your stance when walking or running. "How wide is that?" you ask? Right!
It depends. One of the best exercises I know for identifying your individual, optimal, functional stance is the Pivot Slip. In Pivot Slips, your feet should travel in two straight lines directly down the hill, while your legs and skis rotate smoothly beneath your pelvis, 180 degrees at a time, linking sideslips. This pivot requires that the two legs rotate independently, separately, although pretty much simultaneously, each about its own axis (two separate pivot points).VailSnoPro's inimitable Pivot Slip (and a very functional stance)
I've often described the motion as how your legs move when you stand with one foot on each of two adjacent barstools and turn the stools with your legs. The movements, and the physics behind them, are very different from how you would move with both feet on the same barstool--which would require your upper body to get involved.
You can also simulate the barstools by standing with your feet on two sheets of heavy paper on a carpet. You'll quickly discover that you can turn either sheet using nothing but the foot and leg that's standing on it. And you can turn both at the same time. If you experiment, you'll find that you gain pivoting power when you move the sheets of paper farther apart. When they're too close together, and even moreso when you put both feet on the same sheet of paper, you lose the ability to pivot with just your legs, and you have to start throwing your upper body around or holding onto something with you hand(s). (Try it--these things are easy to demonstrate and to feel.) You'll also notice that when you stand 100% on one foot and lift the other off the floor, your upper body must get involved again to turn the sheet you're standing on.
So it's easy to see how important an open stance is when it comes to actively turning or pivoting the skis. And wider would seem better there. But width also affects balance, pressure control, and edging movements. We need to be able to stand naturally and in balance on flat skis, and to tip both skis to their right and left edges easily, using our feet. Depending on boot setup, too narrow a stance puts the skis on their outside (little toe) edges, and too wide a stance puts them on their inside (big toe) edges. Somewhere in between is "just right."
Stance width affects lateral balance and our ability to move laterally, obviously--when we need vigorous lateral movements, we open the stance wider. That's why tennis players, goalies, and linebackers don't line up with their feet clamped together.
But stance width also affects fore-aft balance and the ability to regulate pressure along the length of the skis. Back to the two pieces of paper. . . . Stand on them, one foot on each, a "comfortable" width apart, and play with rotating them left and right as you balance on your toes, then your heels, then your whole foot. Try to find a balance point that allows the sheets of paper to rotate about a point somewhere near the centers of your feet. This is a reasonable "home base" balanced stance. Now, pull the sheets of paper much wider apart--two or even three feet apart. As you've discovered, you can twist them with a lot of power, but what happens to your fore-aft pivot points? When you twist your feet to one side, you find yourself standing on the toe of one foot and the heel of the other, right? And it would be even harder to stay centered on your feet with ski boots on.
So--back to those Pivot Slips. Everything has to function well in this maneuver. It obviously requires powerful and long-range pivoting movements. But you also need precise edge control of both skis to control the edge release and sideslip. And you need accurate fore-aft balance to maintain the pivot points under your boots, and to keep the skis slipping straight down the hill without veering left or right. Pivot Slips also require an optimal degree of knee and hip flex. Too tall and too short both compromise the range of rotation available in the hip sockets.
You simply cannot do a good Pivot Slip with a non-functional stance. So practice Pivot Slips until you can do them well (expect lots of practice--it is not an easy drill). Experiment with your stance until you find what works best. And there's your answer to "how wide should they be?" (and many other questions!).