Calculating power for two channel setups seems to be fairly straightforward. But, what about multi-channel systems? If watching a movie with expected peaks of 100dB, should all channels be expected to reach 100dB? Or, will the center channel and rear surrounds peak at lower levels?

I've seen articles on how sitting distance from the speakers affects decibel levels, but I haven't seen anything on how the room size affects levels. With all things being equal (i.e., same equipment and speaker distance from listeners), will the decibel level change if the room is larger versus a smaller room? If so, what formulas are used to account for room size?