IIRC, front and rear are 6.5". Rear can also take 6x9s. The front tweeters are easy to take out from the sides of the dashboard and you can glue in a replacement to the existing grilles.
I'm not actually convinced that changing the size of the rear speakers makes much sense. Most 6.5" speakers come out better in reviews than their sister 6x9s. However, also IIRC, the originals are fitted to the rear side cards, and the 6x9s can be easily screwed to the stiffer metal of the body.