What is the ideal stall size?

Typically, a twelve-foot by twelve-foot stall is considered adequate for all horses. The actual dimensions are more like 11’ 6" by 11’ 6" because of standard lumber lengths. If the stall is much smaller than this, there is not much room for them to turn around or just get comfortable. Of course, if you are remodeling an existing facility that was originally built with smaller stalls, then your choices are somewhat limited. Maybe more turnout time is a good compromise. 

If you are starting from scratch, however, then 12 x 12 stalls would be a good thing to consider. Maybe you won’t be able to fit as many horses in the same square foot area, but the ones that are there will be much more content, which could translate into less chewing, kicking, or maybe even less vet bills. Anybody who boards at your barn will greatly appreciate that you had their horse’s well being in mind when you built it. In addition, lumber is very cost effective when a barn is built in twelve foot increments, and rubber stall mats are usually designed to fit inside a 12 x 12 stall without much waste.