When you choose speakers, you spend as much time analyzing your room size and dimensions as you do choosing the actual speakers. When you're selecting a subwoofer, you need to do the same.
Your room will greatly affect how a subwoofer performs. However, before I get into how your room affects your subwoofer's playback, I'll address a couple of ways in which your room won't affect your subwoofer's playback.
In the continuing evolution of subwoofer speakers, the ability to play low (and lower-than-low) frequencies has become a blessing and a curse. It is a blessing because lower frequencies provide that room-shattering, earth-shaking bass that makes you believe a plane really is flying overhead, or that 10,000 horses really are galloping by. On the other hand, with increased frequency range, there is a tendency for the brainiacs to over-analyze the use of subwoofers and confuse the typical consumer.
As a prime example of this confusion, a number of fallacies seem to persist on the Internet about subwoofers. One of the silliest of these is the assumption that if a room's dimensions aren't large enough to contain a full wavelength of a frequency, the frequency can't be played in the room. In other words, a subwoofer in a small room is not capable of playing those super-low, long-wavelength frequencies. Of course, this is false, and really rather absurd. Any frequency can be produced in any size room.
To prove ...