When we say "size" obviously the spacial arrangement should be considered with that. There needs to be a separation between the rear and side speakers, otherwise it is kind of a waste. Take a look at the front 3. You have to spread the mains out wider for the center channel, right? If the mains are only a few ft apart what's the point of the center? It'll help some, but is it really worth the money? When the mains are spread out there will be a more obvious hole inbetween them, so the center channel will really help out. The same idea applies to the side speakers.