have to concur - room size has little to do with the size of the drivers in your monitors. There are LOTS of factors to take into consideration, but driver size is seldom one of them.
Cabinet size can be an issue if it prevents you from finding an optimal arrangement. In my current basement setup I went with the smaller Sceptre S6 because it far easier to place than the slightly larger S8. (yeah, I found that surprising, it was almost as if I crossed some line<G>!)
Room treatment is important, and a 12x16 room (with a ceiling height - just guessing - of around 8 feet) is going to be a problem. On the other hand, it is an easily resolved problem... or at least the math is pretty easy!
But even more important is placement - placement of the monitors AND your ears. Before you trade in those monitors I'd spend some time playing with placement. You will be surprised at the difference it can make.