I think I'm going to toss a curve to you...
After talking to Roy (Delgado at Klipsch), his general comment (belief) is that a larger horn works better in a smaller room.
Seems a bit counter-intuitive (at least to me it does) but his comment was with the larger horns, they will control the sound better than a smaller horn so you will get less sound splashing on the walls/ceiling where you don't want it.
Also, with regard to larger speakers needing a larger room to sound better..... more curves for you to think about.
I used to have some Khorns in my (open floor plan) living room. I noticed they sounded distinctly better when I was standing 30' away (yes, I measured). Later in life, I replaced the Khorns with the Jubilee's. I pretty soon went to that 30' spot to hear how much better the Jubilee would sound there.....only to be disappointed. That "magic" didn't happen.
HUH???
To cut to the chase, in the Khorns, I had some of ALK extreme slope crossovers. With the steep slopes, it took some time (therefore distance) for the three drivers to meld into a single "bubble" of sound....in my case, it was at 30' where you could REALLY tell the big improvement. While in the room with the Khorns, you could sense the different drivers. Standing at 30' it was "one voice" coming from them. It was pretty cool.
Out go the Khorns, in come the Jubilee's.... These are 2-way, not 3-way like the Khorns.
What I found out is that by losing a crossover point, their sound gelled into that "bubble" somewhere around 7-10 feet so they had that "stupendous" sound while I was IN the room rather than 30' away.
If you want my opinion, you could go as large as the Jubilee since it hits several points.... 2-way, aligned signal and large horn to help control the sound.
It would probably sound better in that room than the Khorn, LaScala (unless you went 2-way "JubeScala")
They're monster boxes so I'm not fooled into thinking a pair will find a way into your room.... I just wanted to toss some different thoughts and some of the logic behind them.