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

Yes, about 40 Hz should do it for most music (unless some crazy person uses real cannon in an overture)

and 3.7 Maggies will do a decent job on 40 Hz or so - and... it you think about how thin they are, maybe they are the smallest...

Hey Chris (ct0517) sorry, I just saw your post asking me about Phil Collins. I missed it somehow. Anyway, I’m not that familiar with Phil's playing, though I do know he is, as am I, a lefty. I never got into Genesis (I’m not a fan of Progressive Rock in general) or his solo career, but there is no question he’s a fine player, and singer as well. He has employed drummer Chester Thompson on at least some of his solo LPs, who is himself quite an accomplished player. Their styles are not up my alley, which is that of Roger Hawkins (Muscle Shoals session player), Levon Helm (The Band), Jim Gordon (L.A. session player, Derek & The Dominoes, Traffic), and Buddy Harman (Nashville session player).

OP,  Demo the Kii 3s for yourself.  All of the blather will fall away and the music will fill the room.

Does not quite meet the parameters you posted, but very impressive low frequency extension for speaker type and cabinet size. Smaller footprint then the majority of stand mounts. 

http://www.decware.com/newsite/HORN.html
Scroll down the page for specifications.

No affiliation with Decware

Open baffle, down to 18Hz

http://www.spatialaudio.us/x1-main