Following Master Yang Jun's advice from his DVD, you begin Yang TCC with your knees "naturally straight" and your feet shoulder width apart. Your legs are not stiff or locked, not bent too much at the knee, but naturally straight.
The steps he takes are natural and comfortable, not too extended, not too short. You step out until your feet are shoulder width apart, this is what he teaches on the DVD. You can test this in your bow stance by turning in your back toes to straight and bringing your foot forward to your starting stance. If your feet are still shoulder width apart and your legs are naturally straight after you straighten up, you're right on.
How far do your knees bend when you're in a shoulder width apart bow stance? You need to figure this out for yourself.
That's how "deep" you will be into your stance work in Yang style as taught by Master Yang Jun.
I'm sure no expert on the subject, but the man who is teaches this clearly on the DVD.
I can't recommend his DVD enough. I've been nose first into it every second I can be since I got it.
The Masters lecture on the Ten Essentials is something I learn more from each time I watch it. His "intent" lessons (when he shows an application for each form) are gems for understanding what you're trying to do when you perform each movement of the form. His descriptions are clear and it looks like so much fun to toss Marco around like that....
But I digress.
The Master clearly shows how he requires his footwork, hands, shoulders, elbows, head, waist, hips, stances, all of it, on the DVD.
If you don't happen to have a DVD player, Grand Master Yang Zhen Duo's book has a very clear description of the same thing in it. Louis' translation of Fu Zhongwen's book has the same descriptoin in it, in slightly different words but you end up in the same place.
Hope this helps,