6.5 vs 8 vs 10 woofer

Why then are most all subs 10” and 12” or bigger?

hard to isolate

depends on implementation with many many other factors

I fundamentally disagree with the notion that there is only one approach to any aspect of speaker design.  That someone purports to have determined what diameter is ideal for a full range driver, regardless of other aspects of speaker design, such as type of enclosure and type of bass alignment (bass reflex, open baffle, sealed box, quarter wave back-loaded horn, transmission line, etc), the type of tweeter to be used with that woofer, and a whole host of other factors, speaks to the lack of experience and understanding of the OP.  That this conclusion was arrived at by watching You Tube videos, adds to the folly of this assertion.

I've heard many different full/wide range drivers used full range, or with a tweeter or with a woofer or in three way systems, and there was not one single characteristic that was common to what I considered successful implementation.  Size did not necessarily matter, type of magnet structure did not matter, cone material, you name it, did not universally determine what was best.  Yes, I heard terrific 8" full range drivers, such as the AER BD 2, used in several different systems and the sound was completely different because other aspects of design were different.  The best application I heard of this driver was in a single driver, quarter wave backloaded horn system by Charney Audio (utterly amazing); another system where that driver was used in an open baffle system with a subwoofer was not as successful to me.  The very best system I heard with a wide range driver had a 13" field coil driver in an open baffle (the 13" driver operating full range (no low pass filter cutting off its upper frequency response) with a tweeter crossed in way up high in frequency. 

There are more ways to make a speaker system than there are ways to make lasagna--don't tell me that there is only one correct recipe, even if it happens to be the one I like.

Not this crap again! 
I showed how this guy made sweeping declarations about full range drivers, and that he was fundamentally ignorant of the fact that the brand he was boosting, Voxtiv, makes a sub with a crossover. OOOOOPS! 

Now, he is starting with stupidity of 8" is the ideal woofer. 

"Listening to several YT (YouTube) vids..." What a joke! Remember, this guy is in about a 10x12 room, so his big discoveries are entirely limited by his room. He is one of the most biased persons claiming to be informed on this site. 

"Better volume control with a 8 vs a 10." This is just fundamentally ignorant. 

Who need this garbage?  :( 

I don't know about that OP.  I run

6 12" OB servo subs in 3 columns (GRs)
12 8" MB PP drivers in 4 SAT columns or 2 72" NB columns (Mine)
12 8" planars in 2 74" 420 lb HDF NB Di or Bipole columns (switch flip)
2 AC G1s Tweeters ribbons 6" ribbon
4 AC G2si Tweeters ribbons 2" ribbon (VMPS RMx Elixir cabinets)

36 active drivers

OB = Open Baffle
NB = Narrow Baffle
PP = Phase Plugs
HDF = High Density Fiber
SAT = Stack and Test speaker system

I don't care WHO say it.. More is better when it comes to drivers...

Fully active direct coupled MB columns (8") with full blown DSP, and an active OXO

These 8" drivers work from 60hz to 300hz.. I use 12K Behringers and a 2496. Absolute control. Very good dampening that actually WORKS..
Dampening CAN'T work unless they are direct coupled. NO OX between the driver and the amp. Most folks don't know that...

Getting them ALL to work correctly together, that's a whole different story with this 13 year old project. I'm close.. This year maybe.. :-)

SO 8" X a lot works for me. :-)

Regards