What makes a speaker too big for a given room?

A very large cabinet can become a problem in itself if the room is too small to distance the speakers far enough from each other. They can have huge reflective surfaces (Dunlavys come to mind again) that can mess with your soundstage and imaging. However, this can be remedied with treatment between the speakers.

I am afraid that the entire room should be treated, and this will not be simple. It is not just big reflective surfaces, it is two big pieces of furniture in a small room. Speakers need space and air to breath, and there wouldn't be much of it. All in all, a bad idea.
Japanese may have their ways, it's a different culture.

Stanwal is right on. I own Dyn C1's (monitor) with a sub but there was a day when I wanted the C2's (floorstander) more for its looks and the added bass that I heard at my dealer. He loaned me a pair for just over a week. At first I loved them but when seriously listening something didn't seem right. Turns out because of the nearfield listening the bass was no better than the C1's in the sweet spot (elsewhere is was a lot better). Also with the C2's there was a huge vertical wall of sound coming at me.

As far as a sub I partially agree with Learsfool. But only in the FACT there isn't much below 40hz. I think the key to using a sub is to get it to blend with your speakers and not make it shake the foundation (which newbee's with subs tend to do - myself included). When set up properly you will think the deepest bass is coming from the monitors and NOT where the sub is positioned. Also when set up properly the sub will add to the mids making the soundstage a little fuller. The key with adding a sub is getting it set up correctly which is not all that simple. Getting the timing right is the hardest part. You don't want the output from the sub to lead or lag that which is coming out of the speakers.

That has been my experience - yours may vary

My wife can tell you if a speaker is too big for a given room in a split second, withour even listening to them. I don't know how she does it - it's a gift.

A speaker can be too large for a room for any of the reasons already cited. However, specifically as to low bass and room size ("20hz is 20hz"):

Quarter wave cancellation (reflections of omnidirectional low frequency waves - usually off the wall behind the speakers) causes irregular bass response. This cancellation is a function of the distance from the speaker to the nearest large reflective boundary (i.e. the wall mentioned above). Usually - tho not always - this distance is also a function of room size. You might put a speaker 5 feet out in a 20' room, you are less likely to do so in a 12' room.

As to "20hz is 20hz" (subwoofer vs floorstander):

You can place a subwoofer flush to the wall and eliminate a lot of this destructive cancellation at low frequencies. A 20hz wave generated in free space behaves very differently than a 20hz wave generated at the wall. A HIGH QUALITY subwoofer will also typically (as near as I can tell from published data) - tho perhaps not always - exhibit much less low frequency distortion at any given SPL than a full range floorstander rated for similar LF extension.

So, one reason a speaker is too big for a room is that it must, as a practical matter, sit too close to the wall to allow smooth bass response.

Marty

PS This doesn't mean a subwoofer is "better", just that smooth, clean deep deep response is usually easier to achieve with a subwoofer.