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

My take is that Noble100 (Tim)  is the "smartest man in the village" with educated insight.  I look to see more of his posts.  If the penchant of the OP is excellent bass response in a small speaker, I will offer two alternatives:  Joseph Audio Pulsars or Dynaudio C1 speakers... pretty flat down to the low 40 hz realm.  I recall hearing the Pulsars at an audio show driven by a Rogers tube amp (!) and the bass response from these stand mount speakers was astounding, with gut-wrenching bass response.  Ditto for Dyn's.  The JA Pearls go down to 25 hz, but are +$20K and are hulkingly large speakers.

As Tim cogently informs us, your goals of having smallish speakers with the bass response that is your goal defies the laws of physics.  I personally would not need a subwoofer for the Pulsars or C1's in a modestly-sized room as I am a lover of sublime mid-range response, but the addition of one or two REL subs to a pair of fine stand mount speakers will fulfill your needs very nicely, but still get you response only to the mid 30s hz.   I use a REL T-7 with my Spatial Audio M4 Holograms and it rounds out the LF response extremely well.  

I asked for the smallest that could do it. Nothing impossible about that since some clearly can and do just a matter of how small that can be.

shadornes Barefoot recommendation was a good shot.   On paper clearly  a lot of bang in a smaller package if not quite able to hit the 20hz mark.  

Goldenear Triton Two+.  Plays flat to 16 Hz.  Smallest speaker I've seen that goes that low.  Two powered oval drivers and 4 passive radiators I believe.

BTW Triton three plays to 21 - but you said 20...so the Two+ is my answer.

Of course there is always the *quality* of the bass to be considered. It's not simply a question of frequency response.