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

most things are spec’s +- dB so for example a speaker might be described as "flat" from 20 to 20kHz +- 3 dB. so it could be down 3 dB at 20 Hz which is half as loud at 20 Hz. No speaker is ruler flat thus when most speaker companies state their speakers are flat to 20 Hz they actually mean they are 3 dB down at 20 Hz or even -5 dB down at 20 dB or whatever.  Also if speakers are spec’s in an anechoic chamber their actual performance in a real room will usually vary considerably.

Atmasphere
The smaller the driver, the more excursion it needs and that's your limit. Bruce Thigpen got around that limitation with his subwoofer fan.

Atmasphere - as you probably know I am a big fan of Bruce Thigpen. I consider him a genius. Bruce' sub is unique, it allows someone to feel a helicopter land in their space at 3 hz if they want. Be prepared to replace some windows, with neighbors knocking on your door.

But even Bruce can't cheat Physics. His TRW sub is using the adjoining space next to the music room - as the box !

Mapman
You know, this 20hz - 20khz audio thing.... imo it gets thrown around like car guys say 0 - 60 mph. How much real music even exists below 30 hz anyway ? Sure, we can look at the chart data.
 
http://www.audio-issues.com/wp-content/uploads/2011/07/main_chart.jpg

But it only tells a small part of what is going on. At least on the forum discussions here. For the real answer, imo, you need to talk to each person to find out what they listen to. Some want to feel no loss in DB below 50 hz with their music. Some want it light and tight. This may be due to an apartment room, fragile kidneys, who knows...... but light and tight is not how live music sounds. 
Also
I can tell you from personal experience the guy that is 50, listening to 2 channel music, is listening to different music, and has different priorities, than my 22 year old son. Now I am managing to convert him slowly. I know this because he keeps asking me for music. But he can't afford a house or his own place. His truck is his room, and it uses two JL Audio subs. And his generation likes the bass loud - real loud. Not because he is deaf. This is because they are using the whole truck as a speaker and standing outside of it when they socialize.    

I guess they're out of production now, but a meritorious mention for the Totem Mani-2's.  Remarkable bass out of a relatively small monitor.

I'm just interesting in what smaller speakers people have found are truly good down to 20hz and if so why.

I've heard Mani's and liked those very much.   I recall they had a unique bass driver design.