Barnhill is very unusual. Awful mechanics but high velocity. It will be interesting to see if she can learn to pitch legally and make it in the SEC.
The longer the levers the better.
I have heard MLB people refer to "effective speed". If two pitchers throw the same speed, but one releases the ball 2-3 feet closer to plate, she should have a faster "effective speed". This would theoretically give an advantage to taller, longer limbed pitchers if their leg drive and glide were good. I have not seen this discussed much, but shorter reaction times for hitters due to this can be a significant advantage, and can also make movement pitches even tougher to hit due to shorter reaction times.
The best pitcher in the world (Yuikko Ueno) is 5'9". Above average height, but not nearly as tall as the Americans.