The room size control allows the end user to customize the overall sound of the subwoofer to their particular room and listening preferences. There is no right or wrong setting, it is all personal preference. We recommend starting with the control at the midpoint on initial setup and system evaluation. From there spend a bit of time getting used to the overall sound of the subwoofer. Then if you want to experiment the more you turn it to SMALL, the tighter/faster the bass can sound. The more you turn it to LARGE, the more powerful Movies can sound...particularly if they have strong bass under 30hz.