What are the smallest speakers that are clean and flat down to 20hz?

I think this is something that would require high end PA gear comparable to a touring system, to create any legit reproduction of 20 Hz flat response or not..

Something different Bass Test  for Phil Collins - In the Air Tonight  

https://www.youtube.com/watch?v=D0U5qunTUJk&index=1&list=RDD0U5qunTUJk

Second the Kii 3's. They were one of my favorites at NYAS, I am afraid to say I enjoyed them more than Harbeth MK40.2's... The technology in the Kii's makes them a good deal.

Are you going small because your room is small? Or are you trying to avoid overbearing speakers? If it's the former, you may have a lot of hump and suckout treatments to do with those long waves bouncing around.

What you ask is impossible, as others have mentioned. Get a set of "bookshelf" style speakers or a small(ish) tower speaker and a good sub woofer, REL, Velodyne, etc. When properly matched and tuned, they can best all but the best full size speakers. The wife acceptance factor is usually on the high side. Also do not discount the possibility of higher end in wall speakers for the satellites. The low frequency is more a function of the room, as others have mentioned, getting enough sub woofer is imperative. 

I've managed to achieve fairly flat/clean response in my 15.5ft x 10.1ft room down to 22Hz, but I'm using four 12" woofers and lots of power to do that at greater than 90dB.

What I've found is that interesting musical content below 40Hz is uncommon. Most of what I hear is stage/studio vibrations, an HVAC switching on, a truck driving by, or other low, rumbly noises that were picked up by the mics during recording and not filtered out. While these sonic artifacts are occasionally interesting, they rarely contribute to the musical performance in any helpful way, so I'd probably be better off rolling things off sharply below about 32Hz. Here's a graph of actual measured response at the listening position before and after correction:

http://www.dsnyder.ws-e.com/photos/potn/Loft_Before_After_and_Target.png

Here's a photo of the actual room from the listening position:

http://www.dsnyder.ws-e.com/photos/potn/loft_wide.jpg

I'm confident that I could achieve similar response with somewhat smaller loudspeakers, but I estimate that minimally four 10" woofers or two 12" woofers would be required in total to achieve this level of bass extension without a separate subwoofer.

I had these loudspeakers before the room...they are obviously a bit large for the space, . If I was buying loudspeakers specifically for this room, I would probably have chosen Harbeth 30.1 or Legacy Audio Calibre and would have been quite happy.