Well according to
http://twinturbo.net/ttnetfaq/FAQpages/boseinfo.html
The rears are 6 1/2" and the fronts 3 1/2". Read further and it says that you might have trouble with replacing the stock speakers because of the unique boxes in the doors/rear pillars for them. It was all designed as a system and has to work as a system. And frankly I've found that the BOSE units deliever very clear high quality sound up to very loud volumes but they tend to lack bass. At some point you just have to push more air, which means bigger speakers. Your choice.
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1989 240SX Fastback. . . . . . . . . . . . . . . . . . . . . . . . . . . 1995 300ZX Twin Turbo
Warning: Objects in mirror aren't as fast as they thought they were.