[drem914]

You will only be able to replace the rears with 3-1/2" speakers. It will require some cutting to the plate that currently holds the speakers, if they are the oem units.
You might want to try to just ad some tweeters in series. Many have done that successfully. Without upgrading the amp you will not have much power going there.

The fronts or also maxed out at 4" units with a 1" separate tweeter.

the doors are "subs" you can go larger but will need to cut the baffle that they sit in or make new ones.

Check over on the Renn Tech site. There are many many threads of people that have done speakers and head unit upgrades with a lot of visual and descriptive information.