If you are a fan of having rear speakers(I am not) use the same 6½'s you are using in the front, otherwise you will end up having to EQ the front and rear differently to get the rear fill to blend in properly.

I don't like to have rear speakers at all, I feel that it ruins the image of the fronts. I have 6½ 3 way components in the doors, and a sub in the back, with an 80 Hz split between them.

John C.