When it comes to subwoofers, the answer's yes; size matters greatly. If you have a couple of 8" drivers in a box you're calling a subwoofer, you're going to be disappointed if you follow the setup steps we'll be discussing - because you won't get the results we're getting. Big is better as long as you're not asking it to go too high in frequency.
I am always reminded of the classic film Crocodile Dundee (the first one) when the local tough tries to rob Mick Dundee with a small knife and Mick whips out his big bad blade and says to the tough "that's not a knife, THIS is a knife" with a knowing smile shared between two tough guys. So let me just start of saying to those of you with small subs, "that's no subwoofer, THIS is a subwoofer".
The simple fact is surface area is a big deal for moving air and achieving low frequencies. The lowest notes on a pipe organ,BÃ¶sendorferImperial Grand Piano and a tuba is around 16Hz - a frequency considered below the threshold of human hearing. These instruments do not create single notes at this frequency but what they do generate is full of higher frequency harmonics - the harmonics giving the instrument its sound signature - and those higher frequencies in combination with the low fundamental give a full visceral feel to the note. When played in a live space, any one of these instruments will produce sound you both feel and hear - your stereo system should do the same.
To hit realistic in-room levels of undistorted clean bass at 15Hz is quite a feat. Carver's Amazing subwoofer claims to go down to 18Hz with a 12" woofer, the same claim is made with my current sub the Martin Logan Descents that have three 10" woofers, but I suspect these claims are made in a close mic setting.
The problem with too small a surface area has to do with coupling the air in the room - which at very low frequencies is not linear and as the frequency goes down, the surface area of the driver has to go up rather dramatically to get what you want delivered in the room.
If I were building a subwoofer I'd start with an 18" woofer, or at least two 12" drivers. The total surface area of two 12" woofers is 226 inches, while the total surface area of an 18" driver is 254 inches - so nearly the same - my three 10" drivers are about 240 inches. A single 12" woofer has only 113 inches and two 8" drivers about 100".
Here's something else to consider - a single 18" woofer is better, in my opinion, than two 12" woofers or three 10" drivers because a single piston couples more effectively than multiple smaller drivers. Sub manufacturers like using multiple smaller drivers for three reasons: they are easier to source, have less mass so it's easier to get a faster transient response and their higher frequency performance is better than a single big woofer.
I would counter these arguments by suggesting the sourcing of a driver should be based solely on performance, not economic considerations. I would also suggest that given a proper motor assembly one can have any transient speed desired and lastly, in a good full range setup who cares about going higher than 60Hz? Ok, I got that off my chest and, since I don't make subwoofers, let's move on to the real world of what most of us are likely to own.
So, I imagine nearly none of us have an 18" powered subwoofer around and in all likelihood, we are probably going to have a single 12" woofer per side. Oh, did I say per side?
Yes, sorry, but the myth of the single sub being adequate is plain incorrect. Stereo subs are the only way to go - unless you simply cannot afford to have two. In that case, I fully understand the limitations we all face in this hobby so I'll try my best to help you make a single sub work as well. But by all means, if you're financially capable of having a stereo subwoofer system, then that's the best case for your musical setup.
Lastly, make sure you have a great subwoofer. I'd sooner have one great subwoofer than two POS.