What makes a speaker too big for a given room?

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.

My speakers have side firing woofers. How does this factor into room placement? Does this have any impact on the rules of speaker placement?

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.

So, Rrog, "whether the room will allow the bass to develop properly". I was trying to think along those lines. Should this mean you want your bass produced by smaller drivers? say, 7" drivers? Because typically in larger speakers you're going to get larger drivers for the low end--maybe 10, 12, even 15" drivers (woofers, cones, whatever). In fact, a lot of subwoofers have those larger drivers--10" at least so it seems antithetical to insert a subwoofer into any small room based on that. On the other hand, a small room may not give you enough bass simply for lack of room volume and if you can get enough air movement via the larger driver (subwoofer or no) then it seems that the argument fails. I've got a 11.5' wide by 15.5' long space, open on one side and I'm debating whether I should bring in B&W 803 Diamonds (larger) or the 804 Diamonds (smaller, but still floor standers).