I know that the fronts are 4". I also know that there are 6 rear speakers, some are 5.25" and some are 6.5". Which ones are which? Side panels? Above rear window?

 

i did a search and found these sizes.

Front dash = 4"
Rear cargo floor = 5-1/4"
Rear cargo roof tweeters = 3"

just do a search, and you'll find everything you need to know about your 2nd gen. audio system. i have the same make and model as yours, and am in the process of upgrading my factory speakers.
here's vehicle fit guide link for some infinity speakers. hope this helps.
http://www.mobiletoys.com/partners/...UNNER&YEAR=1991