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

Have older b&w speakers and bass drivers are 7 inches vs 804 d3 speakers that are 6 1/2.

the larger drivers seem to really open the sound stage, more open and less constrained.  Imaging of older speakers not as good but a very enjoyable listen.
Not sure why driver sizes tend to be smaller these days unless you pay a lot more.  Hear that most consumers want more compact mains so drivers are smaller.  Kinda sad.
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....
@emergingsoul --

Not sure why driver sizes tend to be smaller these days unless you pay a lot more.

Indeed, a lot more; size is progressively expensive in hi-fi, not to mention high-end, and for physics to be more closely approximated - taking a small, already expensive package and making it into a bigger one - it’s an expenditure eventually only the very few can accommodate. A shame really obsessing about painting that little corner of the canvas in the most minute of detail, when the rest of it, its sheer scale and totality, is neglected. That is, at least when one knows what’s missing.

Smaller size is by and large about convenience/practical considerations/interior decoration and spousal acceptance, from where effort is often invested to make it appear as if the smaller package is actually a desirable trait, or one overall sufficient sonically (some justification is obviously sought into buying small-ish products that expensive, or even less expensive). And who wants to pay $1,000 for a measly sound bar? Many, it seems; they’re selling like warm bread - go figure.

I’m not trying to negate the potential prowess of small(er) speakers in smaller spaces, but it’s when smaller sized speakers become excessively expensive that bothers me, because vital aspects of the sound only achieved thorough an adherence to physics (i.e.: sheer displacement area, high sensitivity, etc.), important to me, are still sorely missing. Why then bother, and at that price? 

Simplicity topologically (and what it in-effect leads to) is something to aspire to, a trait with smaller speakers in particular, but it’s likely a side effect of a purchase incentive here that’s not always consciously sought to begin with, at least not as much as seeking smaller size per se. Holding on to that trait, however - and for good reason, I’d claim - the question could be how to scale up in size without throwing out the baby with the bathwater, and I’m thinking mostly complexity and price here. It’s certainly possible to achieve, I might add, but not through the package one would usually expect, while also needing a healthy dose of change in mentality towards sound reproduction in a home environment.