Dolby Digital 5.1 Surround Question?

I have an onscreen setup menu on my Surround system. So you go into the speaker setup and select your configuration and it show pictures as you change things around, like woofer or not etc. There is a selection for large ir small rear speakers in teh menu. I would like to pump more power to the rear speakers. OK here is the question: which selection sends more power to them? One school of thought is that the smaller speakers will require more power to equalize the room. the other is that the larger speaker woudl require more power. Which is it? I have tried both and cannot tell the difference so I just wanted to set it correctly and be done with it. Thanks