Good point. Many audiophiles want the biggest speaker they think they can get away with, even though this may not be optimal.
But it depends on what you want. If you don't care about imaging, a speaker too big for the room may be fine. Personally, I cannot stand when a speaker does not disappear from the soundstage, and you clearly hear the sound being partially glued to the left and right speaker. I have a room 24 x 12 x 8.5 ft, and I have a monitor / subwoofer system (which works fantastically and creates a rather large soundstage). I am afraid that, because of the limited room width, a larger speaker will not completely disappear from the soundstage in my room.
Of course, everyone will say, "my speakers disappear". But have you ever heard what this really can mean, and how great it is when it happens? I bet many haven't. Or perhaps if they did, they don't care.
Another thing to consider is coherence. With a larger speaker you often have to sit further away for the drivers to completely blend. In my room a larger speaker would be no problem in that sense, because I could sit further away without compromising soundstage depth (my speaker drivers are 7 feet from the front wall) or having to sit close to the back wall which may bring its own problems, especially with bass. And next to avoiding coherence issues, sitting further away may also be necessary with a larger speaker for achieving the act of "disappearing" from the soundstage. Yet having to sit further away in a less long room may give problems (less flexibility with creating soundstage depth, bass issues from having to sit closer to the back wall).
Yet even though I can, I don't want to sit further away than I do (distance tweeter to ear is 8.7 ft) because I would miss intimacy on small scale chamber music, for example, an intimacy I don't want to live without. With those monitors, I even could sit considerably closer than I do.
I am not saying everyone should buy monitors (even though many might be surprised what a great monitor/sub system can do). Yet restraint with the size of a multi-way floor stander, depending on room size, may be a good thing.