[Zero10]

Late 87+ cars are able to accept 6.5" speakers in the back or 4x6" speakers.
For the fronts if you want a drop-in replacement you will have to go with 4x6 speakers or 4" components mounted on a 4x6" plate.
If you like cutting and making panels then 6.5" speakers up front is quite possible. Many people cut back towards the arm rest and use spacer rings to bring the speaker out enough so it clears the window motor mechanism.
Another thing to consider is clearing the dash when the door closes. It's a tricky fit.