I've seen pitchers who use a short stride and pitchers who use an incredibly long stride. What are the +'s and -'s to each? If you coach pitching which do you teach or does it depend on the kid?
The general rule is that the longer the stride is, the better, until the point that mechanics break down, then back off a few inches.
I'm in the camp that all strides should be similar (I teach my batters to watch the pitchers stride foot to telegraph pitches), but I realize opinion varies widely on this so I'm not going to be anal about it. she should do what her pitching coach tells her to do, and when she feels she's mastered that coaching, she can make her own adjustments.
Flexibility and athletic ability will dictate the stride. There is also a style of pitching (that pre-dates leap and drag) that we now call "step style" where the pitcher does not leap but rather just steps with the stride foot. The theory is that this takes a lot of upper body strength and is better suited for larger "not so flexible" girls, but that too is open for debate.
I usually start with a stride around the same length as the girl is tall, but again, this varies based on athletic ability and flexibility. From there I like to increase the stride until things break, then back off.
My 11yo dd takes a very long stride. She can get almost to the edge of the circle. IMHO, it's great because it puts the release closer to the batter and trims down batter's reaction/recognition time. She is also taught that all of her pitches are to look identical to the batter. The stride length never changes based on pitch. Nor does it move to the left or right based on pitch. She is taught that grip, spin, and release determine what pitch she is throwing and where. I do agree with the above poster that it should be as long as possible without compromising mechanics.
Screwball, are you saying that the old step style caused injury and was not conducive to creating spin? Both of those couldn't be further from the truth. Leaping and dragging sometimes prevents the pitcher from concentrating on the release of the ball and all that stress it produces makes injury more likely than just stepping
Sling-shot is more prone to injury, but step style (which I use) isn't. One of my girls over here has switched to leaping and has had ankle problems ever since.