Everyone you ask will tell you something different. In the end you will still have to make a best guess judgement. What I can tell you though is that typically the center channel is something like 13db down from the LR fronts. That means that it is only getting a small fraction of the power that the fronts are receiving. In this scenario Mtrot's statement is incorrect. -3db is half the power and +3db is double the power. Also, in theater configuration the center channel has no requirement for any real bass so typically you don't need anything large.
