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

Barefoot Monitors are probably the smallest high performance speakers.

The MM27 has the smallest footprint and can compete with speakers many times their size.

Physics can be a pesky thing, and the mutually exclusive nature of including "small" and "clean at 20 hz" in this post does seem a little paradoxical. I use 2 RELs in my hifi rig and they're sort of smallish I suppose, as are the drivers in my Silverline Preludes (3.75" woofers)…I have a 92 lb 500 watt Mackie studio specific sub (in my studio specific studio) with 2 12s (active and passive) that supposedly does 19hz…but it's on the large side. So, no free lunch. 20hz…hmmm…large Wilsons maybe?

Check out Gershman Acoustics Avante Garde. Gets to 20hz or close and is not large at all.

Kii audio model 3s. 

The Excellent Deavialet Phantom is that type of speaker yhat can place with grace 
Or knock or thump you in the chest 20hz test tones confirmed . my brother bought a pair of yhe easy release models from last year thin 2- 6inch pistons in a automobile  like a Ducati twin engine horizontally apposed and and unit is only as big as a soccer ball but can knock out the sound pressures !!