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

It is an engineering compromise, just like almost everything is in life. It is basic physics.

The larger cones move more air and hence provide a much more realistic and voluminous bass. However, the larger the cone gets, the less rigid it becomes, affecting the sound negatively.

If you could find an infinitely rigid material, which would never lose its shape regardless of how much air pressure it was exposed to, then you could build infinitely large speakers. But real life has limits :-)

There is also one more very important factor as to why most drivers (and hence the speakers) are smaller these days.  It is the WAF :-)

Makes sense very large drivers loss some ability to handle detail.  So there is a sweet spot between, say, 7 to 12 inches. And beyond 12, you would be in a subwoofer zone where detail is less important and less feasible.  
Personally, 10 inches on the b&w 800 seems ideal.  
There is only so much tech research can do while trying to cruelly shrink a bass driver and maintain sq.  Physics has its limits
Manufacturers and designers need to wake up and not artificially  suppress quality bass sound to accommodate a smaller cabinet.  How cheap are things getting.  Remember how they trimmed the size of a carton of oj a few yrs ago.

The wharfedale 225's as reviewed by three different reviewers in stereophile, who all agreed that the speaker has phenomenal bass, uses only a 6.5 inch bass driver, ported at the bottom surface of cabinet. One reviewer, I think the late Art Dudley, was taken back by how much useable response the speaker had at 31.5 hz! It was confirmed with measurements. He said he heard what he heard....just one example. I still own a pair and still love them, but My tannoys with 10 inch driver somehow seem effortless, maybe more relaxed. 

MC, I got it!!!! Clever and fun.

More seriously,  size not critical - but only nowadays. Design and new tech are changing the old parameters.

A big sound depends on how much air is moved forwards.  So this meant earlier large cone size vs breakup was the issue. Larger and stiffer=more bass directed forwards.

Nowadays with high excursions being permitted by the new materiels, and some creative thinking, it's possible to get smaller cones (almost zero breakup) to do similar stuff.  

A recent development has been to design the speakers port so the back wave is phased to reinforce the front. Eg D&D, some open baffle designs.

Kii use a slightly diff approach.  They have a back speaker which slightly out of phase so that it cancels the front wave behind the speaker and reinforces it in front of the speaker.   Along with dsp and long throw cones their speakers measure down to 19 -20 in room +- 0.5 dB!

KEF's new approach is to simply cancel the back wave internally - with no consequent penalties.  That is unbelievable in my book but they claim to have done it.

So the answer to your question is that the new slightly larger bass drivers may improve bass response, but you should look at these new technologies first, if possible.

Best wishes

Quite agree AubreyBob.

Back in the day most woofers were 12 inches and some were 15.  Who remembers the 15 inch Fane that could be bought very reasonablly and put in home-built cabinets?  It was designed for use in cheap PA and guitar amps.

The issue with such big units was big lack of cone stiffness.  Yes, they moved a lot of air and delivered a lot of bass but flexure of the cone meant it was very flabby, with poor cut-off of notes and inclined to tunelessness.

So from around the 80s, designs featured multiple smaller units.  Note as a good example the B&W 801, a 'budget' high-end speaker introduced around 1997 with a single 12 inch woofer.  After some years the 801 was withdrawn and replaced by the 802 that has two smaller units.  Reviewers reported cleaner bass with no loss of sound pressure.

More recently the application of new exotic hi-tech materials has enabled the revival of 12 inch cones that have sufficient stiffness to allow low distortion AND big air movement.

So OP, the choice is yours....