I think it depends more on preference than cubic feet of your room.
If you have a very big screen, and don't want to be able to localize the dialog, having 2 centre channels is awsome.
I currently run 2 M3s, one above and one below my 92" screen, the dialog sounds like its actually coming from the actors lips when watching movies. I have M2s on order and should be here this week (in the next 2 days) to replace the M3s.
I will have a hard time going back to a single centre channel above or below a big screen.
