I have minis all the way up to 38" as well as foundation shetlands. My stall doors are just low enough that I can put my chin on them (I am about 5' 6") I know that she now has only little minis, but unless she makes them adjustable, I would not recommemd making them much lower than that cause you never know when she will decide to get something a little larger.
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.
Here are our stall doors and I think they are at 42" at the top. I wanted and open barn that all the horses could look at each other and touch each other . This is just perfect. My tallest mini is 33.5", the smallest (aside from babies) is 29.5". Everyone can look out over their doors except the foals.
My stall doors vary somewhat, but all horses can put their heads out into the isleway. I don't like a solid door, which prevents a horse from interaction with the outside world, so to speak. I have always disliked walking into a barn and seeing a tidy row of walls...give me a horse head hanging over a stall door, any day.
My stall doors are about four feet high, with the bottom two feet solid and the top two feet mesh. The horses love to be able to look out and see what is going on, and it helps for air circulation too.
I'm still trying to decide exactly what kind of stall doors to do for my new barn; I want something as similar to what I have as possible, but the barn style will be different so I may not be able to use the same ones I have now. Decisions, decisions!
I had my horse barn before I had minis, so I had to figure out a way to make it work for minis, I love seeing their little heads sticking out and they love watching what im doing, mine are at 30". I have horses up to 37" and not one has ever tried to jump out.
My barn was built before mini's were in the picture and the doors are too tall. I am in the process of building new doors for them, I think I made them about 33-35" tall. My barn does not have a central walkway so I like my mini's to be able to see over but not put their heads over because sometimes they get a little over anxious around feed time and I don't want them nibbling my little helpers hair!! My girls 3 & 5!!
My old doors were about 48" tall and I find my mini's panic a bit when they are in there!