As a turner gets better at turning and skills increase, the stress and strain on a tenon or recess becomes less. When I first started turning, my tenons were beefy and my recesses deep, now, tenons are shorter and recesses shallower. So, your rule is to start off on the safe side, turn comfortably, set yourself up for success. Your tenons should be as long as you can make them without bottoming out in the chuck, others have mentioned the effect of diameter to jaw size and that should also be taken into account. Shoulder should be slightly under cut so it sits tight to the top of the jaws. I feel that tenons should be cut precisely and with care, not quick and dirty cause it's just going to be cut off later. Same with a recess, turn them clean and crisp. With the jaws closed tight, make the opening slightly dovetailed and just barely big enough to fit over the jaws. The top of the jaws should sit on a nice flat surface, the inner corner should be crisp. A lot of good info in this thread, and even more in previous posts, search them out but don't hesitate to ask more questions.