How much work and how much money outlay are you willing to partake in getting the area ( can't really call it an enclosed room ) to sound good without slap echo and resonances? The sky is really the limit, but having such a large space to energize might make the bass a bit thin.
If you plan on using K-horns, the long wall would be preferable, with the listening position near the railing. False corners or well reinforced wall construction is preferable. ( think plywood ) If you have the option to re-frame and drywall. consider splaying the ceiling some, as this will add in diffusion. Another problem in this space is the lack of "stuff", as soon as you start to add items such as carpet, couches, bookcases, etc, it will aid in absorption and diffusion.
Your best bet would to be to consult a professional that is well versed in acoustical design, for a consultation that I would consider a good investment. ( obviously this is dependant on travel time, your distance from a major center, etc )
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