I know what you mean about being to much for a room, but to me it is more important to match your sub with your main speakers. I have some small polk 2 way bookshelf speakers, and my ported css sdx10 is showing the weakness of my mains.
I would love the bass that a 12 or 15 would provide, but I would seriously need to upgrade my speakers. Yeah, I know you can cut the 12 or 15 down, but why build a 6+ ft box, when a 2 or 3 may do? But then you need to look at headroom, my 10" runs out of steam sometimes. Oh, so many compromises to be made!
My room is close to the size of your room with openings as well. I think a 12" ported would work nice, depending upon placement. My 10" sounds great where I have it located. However, I placed the sub in many locations, and some spots just sounded terrible.