6.5 vs 8 vs 10 woofer

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".

Why the ad hominem attacks on the OP? He's stating an opinion and adding in his personal experiences to support thoughts on the matter. If you wish to refute it, fine but I don't believe there's any reason for name calling. The churlish remarks don't paint one in the finest of ways. And for the record I don't know the OP though I've seen previous posts.
Also, in this case I happen to disagree with him only based on personal experience with a 10" sub. I do agree with several responders that the individual implementation, source material and personal preference would practically dictate which is "best". For my office system the OP recommended 8" woofer suffices nicely whereas the 10" of my full system would be overkill.

In any case, I suggest a bit more civility ladies and gentlemen. There's simply not enough of it in the world IMHO.
Happy listening.