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

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.

I was just going to mention room gain. It will be 3 - 10 db/octave, depending on a variety of things. 
The posts about relying only on anechoic measurements are fine, but they don't translate directly into what you hear in your room.

My REL pair of subs, in my 18 X 35 room, give me flat to 22, down 3db at 20, from an 85db reference. Unfortunately, down 6db at 16hz. And a peak at 28, which I could soften by speaker direction, but I like 28hz (upright bass low frequency).
Luckily, those ratios exist for me at a 65db reference.
In a fairly large sweet spot that covers 4 listening positions. Placement fairly straightforward - optimizing the subs' distance from corners, energizing the room.

I also boost a bit with controls, to accommodate the Fletcher-Munson effect at low listening levels, if i think about it.

I mention these several things, because they are part of the listening experience. Reference level, room shape and size, room gain, floor coupling, preference, size of sweet spot - and don't forget those peaks, which may fool you into thinking that sound levels at your targeted frequency are deficient.

I'd like to be flat to 16, but I also want to be able to move my speakers. In my mind, tweaking sound with placement experiments is thwarted if you have to invite friends to move the monsters.

I like my setup. The RELs are the first subs that mix so easily with my mains. It's not like LISTEN - I HAVE SUBWOOFERS. It's like Wow, what a nice, integrated full spectrum sound I have.

Mikapen
I also boost a bit with controls, to accommodate the Fletcher-Munson effect at low listening levels, if i think about it.

There is no doubt to myself, about this effect. It is very important. For the others benefit I post a link on some info to it.

http://ehomerecordingstudio.com/fletcher-munson-curve/

"
What is it?

The Fletcher Munson Curve is a graph that illustrates an interesting phenomenon of human hearing.

When listening to music through your studio monitors or headphones…
As the actual loudness changes, the perceived loudness our brains hear will change at a different rate, depending on the frequency.

Here’s what I mean:

• At low listening volumes – mid range frequencies sound more prominent, while the low and high frequency ranges seem to fall into the background.

• At high listening volumes – the lows and highs sound more prominent, while the mid range seems comparatively softer.

Yet in reality, the overall tonal balance of the sound remains the same, no matter what the listening volume."