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.
Licensed Veterinary Technician