His question stated "smallest" that plays to 20 Hz.
I’ll grant you that 48" is a tall speaker...it also has some depth but it is a narrow speaker.
Lets have some fun here: Name me a pair of speakers (no subs allowed) that play to 20 Hz that are smaller than Triton Two+. Bonus if they go to 16.
We'll use area (LxWxH) as the "smallest" deciding factor.