What makes a speaker too big for a given room?

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).

Full range speakers with no adjustment for bass loudness in a small room will not work no matter if the bass drivers are 3-7" or 1-15". The reason subs with monitors are suggested for small rooms is you can independly adjust bass output for the room.

There are some full range speakers that do allow adjustment of the bass level but these are basically just built in subs. My speakers have this feature and it makes them much more flexible.

The farther the individual speakers are apart on the baffle the farther away you need to sit from the speaker for the sound to integrate. You need to sit at least 9 to 12 ft away from most 3 or 4 way speakers for the sound to integrate well. Then you also need space behind the speaker or bass will be boomy among other problems.

Regarding B&W 803 vs 804 the 803's don't really put out that much more bass than the 804's. I used to have 805's with a sub many years ago. I have only heard the 803's at the dealer in a large room. That said I think you could get away with the 803's in your room. Just don't try and stuff 802's or bigger in there. The 803's would benefit from a good sub as well IMO. As far as I know they only go down to mid 30 hz. Unless their specs have gotten better over the years.

The fact that the drivers on the B&W speakers are close together on the baffle should help them integrate better in your room.

Sean