I fit 6" speakers into my Civic, but 5 1/4's came out . Go figure. For the rear I'm running 6", but in a "custom enclosure" (Zip ties) however If you remove the speaker housing you can place a 6x9 audiophile plate back there, or make your own plexi speaker plate. Anything up to 6" will work fine. In fact, I may have a set of speaker brackets for sale made of plexi. The guy selling them is a prick, and I just as soon rather not deal with him.
-> Steve