Shouldn't it be "extremely long legs"?
How high must the saddle be to cause hip pain?
No - if you have short legs the crank arms are long in proportion and will move your knee up more.
The saddle should be at such a height that you are not forced to rotate your hips when pedaling in order to reach the bottom of the stroke. Hip pain can come from a variety of causes, including riding too high a gear, seat too high/low or too far forward.