Bass driver size depends on the design of the cabinet and the driver design to utilize that drivers best potential. In other words the components and cabinet they are mounted need to be designed together with an intended sound.
The difference between a 6" woofer and a 6.5" or 8" woofer wont matter. The design specification of the woofer matters. You can have a 6" woofer that has a usable frequency range of 30hz to 5000hz and a woofer of 6.5" with a usable frequency range of 80hz to 5000hz. Just because the woofer is larger does not mean it will have as good of dynamics as a better designed woofer. Either woofer in the wrong cabinet will sound crummy. A cheap $20 Woofer matched in the right cabinet will sound better that an $80 woofer in the wrong cabinet. All depends on the design and quality of materials. not size. Let your ears tell you what is best.
The difference between a 6" woofer and a 6.5" or 8" woofer wont matter. The design specification of the woofer matters. You can have a 6" woofer that has a usable frequency range of 30hz to 5000hz and a woofer of 6.5" with a usable frequency range of 80hz to 5000hz. Just because the woofer is larger does not mean it will have as good of dynamics as a better designed woofer. Either woofer in the wrong cabinet will sound crummy. A cheap $20 Woofer matched in the right cabinet will sound better that an $80 woofer in the wrong cabinet. All depends on the design and quality of materials. not size. Let your ears tell you what is best.