What makes a speaker too big for a given room?

Your wife can tell this immediately because she knows when she doesn't have enough room to breath. She only needs to "listen" to herself to know it not speakers.

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?

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.