Actually, in my experience, you can limit the camera's height by changing the floor/ceiling heights of a sector that's right behind the camera's viewpoint. I haven't done this in a while so it might not work so well, but I've seen it happen. The reason for it being slightly behind the viewpoint, of course, is so that the player never sees this sector.
I don't know if it's possible to make the sector adjust itself to the player's height so that the camera stays that way, unfortunately. It'll just restrict itself to the space the sector provides.