I have heard so many diff sizes for our cars, but i do believe the RS non-infinity system-- front doors-5 1/4, and rears are 5 1/4 or 6 1/2.
Some say you can fit the 6 1/2's in the doors, but i havent even had the time to get my door panel off and do my own upgrading yet. The rear speaker pods can also be cut out to fit 6x9's, or just get the factory 6x9 pods from a infinity system equiped car at a junk yard. My 98 RS also has the dash mount mids/tweeters, but not sure if earlier year RS's do or not, but they are like 2" or 2 1/2".