Smallest room dimensions for good bass to 20hz

Upper corner front left to lower corner rear right need to be 1/2 of 56.6 ft. That length works out to be the SQUARE ROOT of:
[(square root of: L squared + W squared) SQUARED + H squared]

Of course if your listening room has at least one wall which has (roughly) half its total surface area open to an adjoining room, you need to measure the longest top-to-bottom diagonal of the combined spaces ;-)) Have fun!

Kanuk, the space between the headphone driver and your eardrum is not the same as the space in a room between the speaker and your ear.

To vibrate your eardrum, the air in a whole room must already be vibrating at (whatever) frequency.

A headphone driver uses the air between itself and your eardrum as a 'fluid coupling', using it to 'grab' your eardrum and shake it back and forth at the same frequency.

Once again: "The longest room dimension has to equal HALF the wavelength of the frequency you want to produce in it. ;-)

"Is using a headphone the same as listening to speakers in an anechoic chamber?"

Similar, but the headphones are more efficient. Try listening to speakers outside sometime. You practically have to 'wear' them (like headphones) to get any volume ;-)