8 x 13 x 20 seems ideal shape. I would not worry about the room being too small at all!
I think they will work well if you can get them far out into the room (away from walls and corners). These speakers have a huge bass hump (resonance) not unlike the Wilson WP series. If you cannot get them out into the room then I fear you will loose most of the clarity of the lower midrange to the overwhelming resonant bass.
Thanks again for the replies. I feel like my speaker placement and room treatments will allow me to experience what the Nova's are capable of. Its just slightly terrifying to make a significant financial contribution for speakers without knowing if you will be overwhelmed with muddied bass overhang. However, as I mentioned earlier, I couldn't conceivably pass them up.