in my opinion, the center channel speaker is the most critical. it gets a large chunk of sound from movies. if it were me, i would get the most center channel speaker you can afford. there are places to cut corners in home theaer, but i believe your center channel should not be one of them.
i have the Csi5 and it is definitely large, almost 2' wide, i think, and very deep. space can definitely be a problem. but, it has proven to be a good performer.