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.

Transmission line makes a speaker larger.

The cabinet on Barefoot MM27 is Cabinet: 20.5 x 9.5 x 15.5 inches (521 x 241 x 394 mm)
Overall: 20.5 x 10.5 x 17.4 inches (521 x 267 x 442 mm)

it is flat to 30Hz and will play louder more cleanly than 99% of what people have here.

Barefoot MM27 with two 10 inch woofers, two 5.25" midrange and a tweeter in a compact 20.5 x 9.5 x 15.5 inches cabinet are the closet thing to what the OP claims to want. It is like a bookshelf two way but having two JL fathom 110 built into each speaker - that is four 10 inch woofers!!!

These will easily outperform Ohm Walsh and any other speaker suggested here in terms of low end response, dynamic range and overall clean extreme high SPL - they are without doubt the smallest truly full range speakers on the market.

There are some nice speaker suggestions above and Totem, for example, make great speakers, as do Ohm, but in this rather stringent requirement of smallest and full range there is a clear leader that makes other speakers that "sound big" look like kids toys.

@mapman  

There is no need to boost everything to flat of equal loudness as music is produced for human hearing in the first place / so it is balanced accordingly and a speaker should be flat overall.

One issue with flat to 20 Hz is that rooms give a natural 6db per octave boost as you drop below about 40 Hz. So ideally a speaker should have some roll off if it is to perform flat in a room. A speaker flat to 16 Hz like the ohm f-5015 is likely to sound awful in most rooms without EQ.