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

Shadorne I think we are saying the same thing. 

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.