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

^That's why speakers should be anechoic spec'd. It gives us a standard neutral base line. Now you just need to provide us with your specific criterion, so the equation can be completed.   +/- 3 dB seems to be fairly standard, though better is well, better. :-)

3 dB at 20 Hz which is half as loud at 20 Hz

To be audibly half as loud is usually considered 6-10db on account of the ear being on a logarithmic scale.

I really mean  the speaker does not roll off much or at at all at 20hz in a typical room.  How flat it is or not will largely depend on room acoustics from there right?  If it can only be done in a smaller room fine, just say so.  

The room will have some influence certainly, but if you really want 20Hz response **with any sort of dynamic range**, its not going to be a small speaker.  Note emphasis.

Headphones are small and can go down to 20hz.  They're pretty small speakers.

atmasphere
3 dB (down) at 20 Hz which is half as loud at 20 Hz
To be audibly half as loud is usually considered 6-10db on account of the ear being on a logarithmic scale.

actually decibels are already logarithmic; hearing is not on a logarithmic scale. the microphone used to measure the SPL is a listening device and measures what the human ear actually hears. If the SPL is 3 dB down at 20 Hz then you will hear 20 Hz as half as loud as any frequency that is measured as 0 dB. If 20 Hz was found to be - 6 dB it would be 1/4 as loud as a frequency at 0 dB.

Yes 20 hz and dynamic range which is why I tend to think larger drivers and speakers for that.   But how small can they be?     

Zu has yet to bowl me over in demos in regards to having that low end dynamic "meat on the bones" as I like to say but demos to date have been less than optimum conditions most liklely?