As high up as you want them. If I were putting them on under an eight foot ceiling, I think I would put them on about 30" high. If you have 10' ceilings or something, you might want to go to about 36". They started to save the walls and tables when people moved furniture out of the middle of the room when it wasn't in use. If you were not eating, the dining table and chairs were moved to the side of the room. So if you make them the same height as your dining room table, you will be OK. In the Amish communities, chairs were hung from pegs on the walls and they were put on to protect the chair legs from poking holes in the walls, hence the name.
There was also a period--much later, when there were murals painted above the chair rails and another period where the murals were done with wallpaper. Of course, if you are planning this you will have to work from the dimensions of the paper. Henry