2.5 Acres enough??

Where I live you are allowed one horse per acer.

I would go outside on your acreage and draw out the size you want on the place where you want to see it. This will help you get a better idea for how it's going to fit your space. After looking at just how much room the bigger plan will take up you may want to check out how the smaller one would fit.

As for what to do with extra stalls, it looks like the floor plan for the barn already has a tack room and wash rack, so the extra stalls could be hay storage, general tool storage, you could open up 2 stalls into a larger stall in case of illness or injury requiring stall confinement for a prolonged period. As someone always said, you can always find uses for more space.

Where I live they perfer one horse per 1 acre :)

I think the general rule is one acre per horse (which is what we started off with)

We've since gotten the adjoining six acres added on to our origional three acres. It really makes it nice to be able to rotate pasture.

Someone mentioned the amount of sheer room that is needed just for equipment and hay storage. Man are they ever right! LOL
You may want to add on to your first floor plan on one end just for this type of stuff. Then if you plan on training your own horses a spot for a good sixty foot round pen is a must in my humble opinion.

I do like the idea of the apartment over the horses. Just be sure to include a good fly control system in your plans for the barn area.

The living area over the barn is a cool idea but like Harley Heaven mentioned, the only drawback I can see is pest control. Our horses are not really close to the house and the flies are a real nuisance. Every time you open the door 3 or 4 get in.

In our county there is no planning and zoning so there are alot of small acreages with way too many animals on them. We know of people with 7 or 8 horses on 3-4 acre properties, which means it's more like 2-3 acres if you allow for the house and outbuildings. IMO that's just too many animals in a small place. They have no pasture and the horses are in dirt and mud all the time. This year the mud has been awful because we've gotten enormous amounts of rain so far. And the flies, ticks and other pests are horrible too because of the wetness. Not to mention the hay bills they have because they have to feed alot of hay year round.

Another thing to check on is insurance for the home. A barn with hay storage specifically is going to make the home owners insurance higher.. While I love the idea it may also be a bit noisy at times with horses below.. I can understand the desire to save the footprint(ie area of ground used) small, it may be a bit costly in the long run.. definitely check zoning, insurance, and all those legal things out before you get too set on building.. :)