Bass Driver Size - how much better is an extra inch?

Cabinet design and driver excursion  more important than diameter.

I've never owned speakers with less than 10" bass drivers but I suspect a dual 8" driver arrangement would be fine for even larger rooms.  (3)8" drivers, probably better though.  Obviously, implementation is key.  When I went from dual 10" drivers to dual 12" drivers, the difference was subtle in my 16x28 listening room.  A little deeper, yes.  But the 10" were certainly plentiful.  I love being able to "feel" the music.  I have a hard time believing 6.5" "bass" drivers would give me anywhere near the same sensation.

The half inch, as a single metric, is irrelevant.

Much more important is the motor quality, room acoustics, and integration.

I know some 6 1/2" drivers I like a lot more than some 8"s. :0

Room fullness makes all the difference in the world, and with no distortion these simple things will rank a speaker very highly.  

High end brand names take notice.

Im not sure that Im tracking that last post...??

Anyhow, yes, 6” drivers, especially dual, can produce a good amount of bass if high quality.  But a high quality 6” driver vs the same high quality 10” driver, and the larger driver will put out more bass.  This is just physics, right?  More cone surface area...