edt asked...I will answer. Physics you say? The average human female and male have some glaring proportional differences that really matter, SPECIFICALLY from a physics standpoint. You can look all of them up like hip width (which creates a lever arm), center of gravity (lower in women), and, not really last as there are lots of others - foot size.
Foot size, as in length toe to heel or shoe number size is the most important and even affects men with smaller feet with regards to board selection. The foot length is the lever arm that applies the force to the board. The shorter the lever arm, the more applied (by heel or toes) force is needed to resist a given force produced by a set board width. And since women's feet are on average shorter heel to toe than men, they do not have the leverage needed to comfortably edge a board that larger footed person can - even if thet weigh the same or are the same height.
I could go on and on about how you can compensate for this with a larger board, like moving the foot pads closer to the heel side edge, but all of those are trade offs with something else like making toe side impossible.
If you are a man with large feet, you may never have experienced this example of body physics since almost no board is made that is too wide for you. If you have been on a board that is just too wide and you can't hold down the edge, you have experienced what most women and men with small feet have to go through.
But as I said before, you can still make it work. Your speed, range (kitesize), and control are all affected, but typically not detrimental if you are skilled. In my previous post, I was in no way, shape, or form suggesting a beginner ride a blow-up-doll. Try to get the right board for her, sized for her (and her feet, large or small), and get working on techniques.
As far as tall people needing longer boards, well.....that has more to do with body geometry, but still a good deal to do with physics. A longer board allows a wider stance with the same amount of board length from foot pads to board tips. A taller person may not feel comfortable having a narrow stance as it does not allow as much torque to be applied since their legs are in more of a "skinny triangle". A shorter person would have a much more squat position that allows the application of more torque to the board being in a wider triangle. Simply put for a taller person to have his legs at the same angle as a shorter person, their feet need to be placed wider on the board. For that taller person to have the same feeling on the board, their feet still need to be the same distance from the tips of the board - thus necessitating a longer board.
A thought experiment to help you with this is to take a 20ft tall person that weighs 170lbs and try to put them on the board you ride. Not going to work, is it? As you shrink that person to 10ft tall, they may be able to ride your board if they put their foot pads at the tips of the board. But this fix would produce some really bad effects, like trim sensitivity that would have the poor guy nose diving with every forward yank of the kite. This illustrates how foot distance from the tips of the board is just as important as stance width.