I went to an SFG course earlier this year and they said as narrow as possible is best. It's obvious not too narrow, as you'll bang your legs up, and it's wider for bigger and/or double bells. Reason: wide stance reduces the glute contraction, making a less powerful swing.
The way they showed to find your stance is to jump up a few times, bringing your knees up, and wherever your feet land is about your ideal width for a single-arm kb swing.
Hope that helps!