6.5 vs 8 vs 10 woofer

@mozartfan  
I suggest that you study a bit about speaker design.  There is a lot more than spec's, but they do tell a basic story.  Look at the spec's on drivers that you are comparing.  
First Mass MMS... The amount of cone & suspension weight will definitely affect the sensitivity as well as how low a driver will play.  
Take a 95 db woofer that goes down to 50 hz add enough mass and you may get this thing to go down to 40 hz.  The problem is, when you add mass, the motor will have a tougher time pushing the mass and sensitivity will drop, in this case, it could be 5 or 6 db.  
Second Xmax.  In the case above, if the cone does not have enough excursion limit, it would easily bottom as mass is added.  Xmax is important. 
Third FS.  This tells you the resonant frequency that a driver plays on its own without a baffle.  It will also give you an idea of how low a drive may be able to go (depending on box)
Fourth QTS.  This tells you what type of box the driver is best suited for and how that driver will play bass depending on the type of box.  
Fifth VAS.  When tied into conjunction with everything else above, this will tell us the rest of the story of how low and how loud a driver will play as well as how flat its response is in a given box. 
In general (not cast in stone)  The larger a woofer is,  the lower the FS is and the higher the sensitivity can be.  
Still,  I have actually had an 8 inch woofer that we modified heavily, but in the end, this driver would play an F3 of 22 hz,  BUT, it ended up being 83db sensitivity and didn't have enough XMAX to drive it very hard.  
I am listening to right now, a 15 that is 95db sensitivity, but only goes down to 38hz.... Try to research the amount of air a 15 moves vs a 6 inch, even though both may go down to 40hz, what does driver size mean? Everything is a trade off, if I didn't add so much mass to the 8 inch,  it still would have gotten down to 32hz and gave me more like 86db sensitivity.  Look at these spec's,  do some research, get a basic understanding and you'll be able to make a better choice on drivers based upon your needs

Here's a kit for you to build.
https://www.madisoundspeakerstore.com/2-way-speaker-kits/seas-a26-10-2-way-kit-pair-based-on-the-classic-a25/

timlub,

That is a pretty nice and concise description of Thiele-Small parameters.  Of course design and implementation are vastly complex matters even when one stays within the proscription of these parameters.  

@larryi    Oh Larry, you are absolutely 100% correct. I was trying to at least give him guidance on where to look for a basic understanding. He would never design anything from that brief description, but it is a start. Recently, I've been playing with how frequency response changes with impedance compensation. Where in the world does that fit in this discussion or how do you get there..... Just 1 step at a time.  I was trying to give him the first step.  

Too many permutations here to have a valid discussion. For example, a bi-amped system with electronic XOs allows for a dedicated amp to control the bass driver and not need as much passive XO at the speaker level. 
Then there is amp design, driver quality, damping factor and slew rates, etc.