We just built a 36x36 pole barn this spring that has room for 4 12x12 stalls, a 12x12 tack room and a bit of feed storage. For now, we have two stalls and tack room on one side and the other is open for equipment storage, but we can add 2 or 3 more stalls at any time. We also put in 2 10x10 sliding doors at each end so a hay truck or tractor can drive through. This is a simple, but efficient design.
Here is a picture of the stalls and tack room before we finished the floor. As you can see, we purchased prefab freestanding stalls and mounted them to the walls, which worked really well.