How big should a horse stable be per horse?

For a professional stable there should be-

1. stall or shelter per horse. Shelter must be at least 12x10 ft per animal. If covered shelter and not enclosed the horses can have 10x10.

2. Feed storage is usually small for grain about 4 sq feet while hay varies depending on size.

3. Min. law requires .25 acres but most counties require .5 acres. The best is 2-4 acres per horse so you can divide and rotate pastures properly.

4. Misc. space for washing, grooming, compost, equipment rooms, etc. generally take up about 50-100 sq. feet. Riding rings can be anywhere from 20x50 to acres of eventing course depending on discipline. A horse in the ring should be able to ride on the straightaway for a good 5-7 strides before a turn. If jumping the ring size goes up.

Stalls should be a minimum of 12'x12' for an average-sized horse.

I personally prefer 14x14.