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

No small loudspeaker can output 20hz flat. And if you designed one that did its output would be so low to be of no use. 

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.