Yes, a 24"-30" wide deck around the four walls leaving at least a 4' X 12' open middle space sounds right to me. From my experience in my 12'8" X 11'8" space I would go no wider that 24" because that open space is valuable also.
That standard-size office chair fills a lot of the open space, especially when an old fat guy sprawls in it:
This view is looking West at the 11'8" wall. That is a 1X6 shelf along the right wall in the pic. Decking on that wall could be wider if I didn't want the futon:
While wider in the corners the decking in front of the windows is only 20" wide:
As has been discussed, O scale takes a LOT of space.
I would do mock-ups to see how best to use your space.
