Think two, rather then one large. Two ten inch, front firing should do the trick. For a discussion of 'two' check out floyd toole's 'sound reproduction' book. I have two 12 inch NHT units and have been able to verify that when only one is used, the 'coverage' is not as great as using two.
Rules for matching subwoofers to room size
I am setting up a new music listening room and will be adding a subwoofer for movies and (mostly) to fill out the full audio spectrum for music. Is there a rule of thumb for subwoofer size/output for a given room volume? My room will be about 11x20x8 WxLxH. Thanks in advance.