Hammer - I have the Studio 20's in a large room (18' x 18' x 12' open to other rooms) and they fill the room for me. They are very robust speakers and with a sub should be more than adequate in all but the largest of rooms. That's my experience. To me, the 20's imaged better and that was more important to me than that fuller sound. That said, larger speakers (Studio 40's, Monitor 7's and 9's, etc.) will sound more full and will fill a large room easier. It's probable that, to most people, particularly those who prefer HT over music, a larger speaker would give a more satisfying fullness of sound.
By the way, when I was shopping for speakers, I narrowed it down to the Studio 20's and Studio 40's. I preferred the sound of the 20's so my dealer offered the following: purchase the 20's and if, after ten days, I wasn't satisfied, I could return them for full credit toward the 40's. If you decide to go with the Studio series, you might consider asking for such an arrangement. Also, just as a point of reference, at an authorized Paradigm dealer, I got 12.5% off the 20's, 23% off the CC-470 and 18% off the surrounds (SA-10R's). That works out to 17.5% off of the package. IMO your dealer may be able to do better on his prices.