6.5 vs 8 vs 10 woofer


IMHO I think the 8 is the ideal size cone for mid/small sized listening rooms. Even for large size listening rooms. 
The lower mid hz's seem cleaner/clearer. And  there is so little fq's in 905+ of the music we listen to, it seems to me the 8  driver is the most perfect size cone.
A 10 size cone  has the potential to become overwheling, aggressive, attacking when amp gain is pushed just a  tad too much, = Better  volume  control with a  8 vs a 10. 
The 6.5 misses some of that lower bass which a  8 can voice superior. 
After listening to several YT vids with a  10 FR, I had considered going 10, but i think  staying with a  8 avoids regrets. 
I listen at low/mid volume. 

mozartfan
My 10" tannoy dual concentric driver sounds good to me with none of the negatives you profess. 
My 10" tannoy dual concentric driver sounds good to me with none of the negatives you profess.
I own 2 pairs of them 12 inches with no negative to my ears for 40 years....

Alas! sold....

Too big for my desk....
Nonsense.  It depends on several factors bu is certainly not as simple as you suggest (or often even in that direction!):
1. rollover frequency - lower means less coloration due to cone distortion2. Area - Pi*R^2 = more is better to move air; more area also means less excursion required = less distortion of some types3. flip side, more are means more flex --> mitigated if not being used as a midrange - back to roll off frequency
A badly designed speaker is just that.  For smaller two-ways the trad-offs are major and you will have issues in the midrange if large, no bass if small.
With a properly designed, hgih quality drivers, well-amped 3 or 4 way speaker, bigger is, within reason, better.  Bu that is all things equal, which they rarely are.
G
If you want bass, use a 24" Hartley in an appropriately designed box.  Last one we built for Mark Levinson's HQD system was about 5' tall, 2' deep, and 3' wide made from 1.25" press board.  It was on 4" casters as it weighed a few pounds, but if you power a pair of them right, you will get some bass in your room that your friends and neighbors will not have.

Cheers!
Like just about any audio equipment assessment, the answer to what makes a perfect woofer diameter is: "it depends".