Great looking room you have there, thank you for sharing!
You have a large overall volume of space to fill, so I would suggest going with a 12" subwoofer at a minimum. VTF-2 MK4 is certainly a good choice, and if budget is flexible, you can even consider VTF-3 MK4.
How much flexibility do you have in terms of placing the subwoofer? In general, it is best to place the subwoofer as far away from those large openings as possible.
Pete - Hsu Research