My barn has stalls that are 12x24. The barn was an old polo barn and the stalls were tiny, so the current owner knocked out the middle wall and made two stalls, one big one.
My 17hh horse loves his stall. He has plenty of room to lay down and stretch out or shuffle around and make dirty faces. No horse has ever been cast in these big stalls and even in foul weather or in injury they don't go crazy in these big airy stalls. In my opinion, horses should always have bigger! My 16hh horse loves the big stalls too, because they can roll to their hearts desire.