Building a barn is a smart idea. Alhough most equines prefer to be outdoors, having covered shelter is important during periods of extreme weather (including lightning storms), periods of illness or lameness, or when access to turn-out is limited.
Concerning your question, stalls should be sized according to the size of your animals, and also according to the particular usage of the stall.
For example, if you own a cold-blood draft breed, you will need rather large stall sizes for all purposes. For a stall to house draft breeds, you should probably plan for 16 feet by 16 feet at a minimum.
If you own warm-blood types of horses such as Hanoverian or Belgian Warmblood. a good stall size is 14 feet by 14 feet. If you own a hot-blood type of horse such as an Arabian or Quarter Horse, a good stall size is 12 feet by 12 feet.
Ponies and miniature horses and donkeys are adequately served with stall sizes ranging from 10 feet square to 8 feet square.
If you breed, and own stallions or mares, the stall sizes will need to be larger. Stallions do better with a larger stall to help them cope with the stress of being an isolated breeding animal. Mares need a larger stall for foaling, and for housing when with a nursing foal.
Remember that horses are healthier and suffer fewer behavior problems if they are maintained on pasture. If a stall is where your horse lives all the time except for occasional rides, opt for a larger stall. In this situation, daily turnout is highly desirable.
Here are some other important considerations for your barn, based on experience and best practices:
- Stall floor - always cover the stall floor with rubber mats designed for horses. These mats are more comfortable for the horses, and make cleanup easier.
- Feeders - if you use mats, forget installing feeders. Horses are more comfortable eating off the ground (as long as they don't ingest too much sand - hence the stall mats).
- Waterers - horses prefer a bucket over automatic waterers. An added benefit is that you know that your horse is drinking adequate amounts of water. The downside is that there is more labor involved.