Speaker size may not be the problem, but the depth and/or the amount it sticks out of the front (protrusion) when installed in the door, thus hitting the back of the door panel. You should find this out first. Crutchfield used to have all these tolerances per vehicle. I have a 95 SE, but for the most part the same system. Check behind the glove box for the amp, to see if you have premium or not. If it was on your sticker, I bet you have Premium.
I replaced all (4) of my speakers first, and there was a big difference, much more bass. I would have stopped at this point, but the new head unit and changer were already being shipped to me and I already had an aftermarket amp. This is what I would recommend first, if it is adequate enough for you then stop. If you decide to change the head unit only, you probably will not see a big difference with just using the head units amp. If you add a four channel amp with the new head unit and speakers, you'll see a big difference. Obviously, the more you add, the more money you spend.
P.S. I bought an older version of the Pioneers that fit in all doors. There was a change in design in the Pioneer speakers, that I believe limits Contour installation, but not sure.
Hope this helps.
