All small vs large means is use bass management or not. While I'd prefer more capable subs than the ones you have, especially in such a large room, yours may well have more capability than your speakers (as well as the subs' advantage of being able to be positioned better for the frequencies involved than your speakers) so I'd start with using bass management and set your speakers to small. The difference in avr power is trival, less than 2dB (and won't matter for the channels using external amps either).