The width of the stance should get gradually wider from the shortest club all the way up to the longest. Starting with your shortest club, a wedge, the outsides of your feet should align with the outside of your shoulders. Typically, your wedges will all measure the same length, so no real adjustment in stance width is needed until you shift to a 9 iron and beyond.
I was taught from an early age by my dad the ball position philosophy adopted by Jack Nicklaus, with our own twist. It is simple and great for consistency. Jack believed you should play your ball position off the instep of your lead foot and your trail foot should widen. So, for a right hander, your left foot would lead and your right foot would widen. I love this theory. However, with your irons, so long as the position is forward of center and always remains constant in relation to the left foot, you are in good shape. As the club gets longer the trail foot gets slightly wider to counteract the length of the club. My ideal ball position aligns my left ear with the ball, which allows me to optimize my lean and launch. Any further forward and I have a hard time keeping my hands ahead of the club at impact. Any further back and I won’t be able to create the loft I would like, particularly with my long irons.