I have 5.25 coaxials in my rear. I like the idea of ebing surrounded by sound instead of being imaged. Some like the idea of being at a concert and having the music go towards them like at a concert, while other people ,like myself, like the idea of being on stage , so to speak, and being surrounded by the music.
Either way, I wouldn't go with any 6x9s. 6 1/2 is the largest I would go but I like my 5.25s just fine.