My doors are just above the withers of the horses, so a 30" would have around a 30" door. I have a wire screen I can put up if they are naughty but rarely have to use it. A wire, see through door can be lower as for some reason they don't seem to try to jump things they can see through. A see through door too high to put their heads over, IME, encourages stall walking and weaving. I had "stallion bars" on the inside of all the stalls- a let down bar, or slot in, that is slightly higher than the door stands back from it by a few inches and will not give if the horse pushes it. Most doors open outwards and the latch can be forced by a determined horse!! I don't use them much now, but I do put them down for a youngster or my younger Stallion.