First you need to figure out how big the finished room will be. Are you adding additional walls, a new floor, keeping the room the same shape? Will the final theater include a bar section with seating or will you use home theater style seating for the whole room. You might also want to leave an area to build a separate equipment room.
Also, acoustically a square shaped room isn’t idea, so you should consider altering the shape so it’s longer than it is wide.
That said, you’ve got space for a very large screen—120 inches or larger. Exactly what screen size you select will in part be determined by the projector you go with, and the seating arrangement, but with that amount of space to play with, your options are wide.
As for the sound system, I suggest you wire it for 7.1 with the expectation that you’ll use two subwoofers to balance the bass better.