I should really stay out of this one. But, I will interject one statement from Wikipedia regarding quadraphonic sound systems.
"Reproduction in the rear speakers should be of the same quality or almost the same quality as the front speakers. Ideally, it is preferred to use four identical speakers." The reasoning is obvious, at least to me. Look at Bose systems. They are tiny speakers providing some of the best sound. Size really doesn't matter. What is most important is the sound balance so all speakers reach your ears equally, with the same sound quality, preferably with four identical speakers.
fade to the front speakers a bit and front vs rear sounds the same. this would need to be done even with the same size speakers all the way around as the rear speakers are so close to your ears. that is if you desire to be able to hear the front speakers.