The rear (as others have mentioned) are an odd 4.x".
when i replaced mine in my old car (95 318is coupe) i removed the speaker and it's box, then made a mounting plate for a 6x9 and sealed it. the 6x9 is a good fit for the hole that the old speaker box will leave. i used the boot as a free baffle and the sound was very good for the money spent..
Another option would be something like this
http://thebassbin.co.uk/shop/product...ts_id/337.html
or
http://thebassbin.co.uk/shop/product...ts_id/339.html
(the second one is basicly what i did).
Yet another option is you could run a pair of 8" woofers there in stealth boxes like this
http://mobile.jlaudio.com/products_s...l.php?fit_id=2