Depends
Hello,
in order to answer that question, you have to define the peak SPL that you want to achieve, and the cone excursion (Xmax) that you wish to tolerate (arguably, the lower the better for the quality of the reproduction).
The equations that govern this are:
Power(acoustic) = 10^[(SPL-110)/20)]
Sd(m2) = Power * 4*Pi / [(2*Pi*Freq)^2 * rho * Xmax)
For instance, if you impose the following rather strict requirements of:
Peak SPL = 110 dB/W(1m)
Xmax = 2 mm (= 0.002 m)
you get:
20 Hz: Sd = 3300 cm2 (i.e. effective diameter = 65cm ! - here's what Fostex's nominal 80cm woofers are for)
30 Hz: Sd = 1470 cm2 (~ the typical 18" woofer)
40 Hz: Sd = 830 cm2 (~ the typical 15" woofer)
and so on (for 100Hz "bass", a nominal 6.5" unit suffices ;-) )
Of course, if you relax the SPL or Xmax requirements, you may get away with using smaller woofers (but at the cost of decreased realism, both because of lower uncompressed SPL and because the larger the excursion, the higher the IM distortion and the less linear the BL and other T-S parameters).
Hope this helps.
Marco