What makes a speaker too big for a given room?

I would also think that big speakers in a small room take up a lot of acoustic space and interfere in a not quite predictable way with the sound they themselves try to make.
So I would say that this is not a good idea. Now how big is too big?

Stanwal is correct regarding the distance between drivers requiring a farther listening position to get a proper blend of the drivers. Dunlavy speakers (SC-IV and larger models) are a good example. Another consideration is low frequency response and whether the room will allow bass to develop properly.

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