sub sizes 8, 10, 12, 15

Woofer sizes, like much of car audio, are about compromise.

Larger woofers play lower and louder.  They generally require larger boxes and transient response tends to be reduced with larger woofer sizes, as there is more cone for the motor to control.  Of course, subwoofer design and box construction play *very* large roles here.

Smaller woofers not only require less space, but they tend not to play quite as low or as loud when plied with the same amount of power.  Again, motor construction plays a huge role here, as well as box construction.

In terms of power required, different size speakers in the same model line tend to require the same amounts of power, as the motor structure is typically the same and things like heat-dissapation and what-not are not really affected by cone size.

aznboi is correct here.. there really aren't concrete answeres without comparing specific speakers and specific enclosures.

^^^ a little clarification here. A 4" speaker can play the exact same low bass note as a 15" driver can, it just cannot do it with the same amplitude. Smaller woofers are most certainly also capable of some devastating numbers for SPL and for low frequency extension. Your statement of "they tend not to play quite as low or as loud" is far from accurate here. An example of this is the Eclipse titanium driver in a 10" vs. say a basic brand 15". The Eclipse will school the basic 15" for output, tonal response and all things being equal for the box, more than likely in low frequency extension as well. There are many factors at work with subwoofers, generalized statements most certainly do not apply.