how big is a large room? specifically, how big does a room have to be to need qs8s as surrounds?

what are the sonic differences between the qs4 and qs8? i know the qs8s extend lower, but do they sound smoother, more detailed, etc...?

if the differences arent that great, and if the room i plan on moving my HT to is considered large, then i can buy the qs4s and save some money for stands.