Sub output: Is it the woofer size or the rated RMS

>A driver with twice the area (say a 15" driver versus a 10" driver) can play 6dB louder. A longer stroke often goes with the larger diameter; if that doubled from 12 to 24mm you'd get a total of 12dB more output at low frequencies given enough amplifier power to use the excursion.

Also note that since drivers are rated based on total basket diameter and that doesn't grow as much with the rest of the driver the surface area differences are greater with smaller drivers.

An 8.5"/22cm driver can have nearly twice the area of a 7"/18cm driver. (220 vs. 126 cm^2)

I'm not sure if this is relevant, but the flagship Ampeg concert bass guitar amp/speaker cabinet is one of the most popular among rock musicians.

The amp does 300w @ 4ohms. The cabinet has 8 x 10" woofers. Low FR is 58Hz @ -3dB and 40Hz @ -10dB. SPL is a thunderous 130dB.

Here's what Ampeg has to say about choosing an array of 10" woofers over larger woofers:

"Why do we use eight 10” speakers? Because we learned early on that 10” speakers work much more efficiently than fifteens or eighteens—and if you put eight 10” speakers together, you can move a much larger column of air. In fact, you’d need five 18” or six 15” speakers to move as much air as the SVT-810AV! And they simply wouldn’t be able to respond to transient peaks as quickly as our tens."

"And they simply wouldn’t be able to respond to transient peaks as quickly as our tens.""

Smaller dynamic drivers do have an inherent advantage in regards to transients over larger drivers even if disadvantaged in regards to reproducing the lowest octaves.

I've always leaned towards smaller drivers in conventional dynamic speaker designs in general as a result.

For subs, I've heard it done well with both larger and smaller drivers, again depending on the robustness of the overall design more so than just pure driver size.

Sometimes it's simply a matter of Proportion, like sticking a babies head, on a 250# man. Or building a huge state of the art hometheater sound system, and then watching the movie on a 27" screen....it's just not right.

The design is of the utmost importance for a sub it's true. It's just easier as the driver size increases. That being said a system with multiple small drivers would probably be the best but would require a lot a of equalization and power to have real low bass.