I wanna say the stock speakers are 6.5, but i cant quite remember...
whatever speaker you get, they don't just drop in there. you will need a bracket to mount them.
the stock speakers have clips that snap into place on the door. you might be able to get away installing 5.25's w/o a bracket, but you will still be screwing the speakers into new holes in the metal. i personally used 6.5's + brackets, so somebody please correct me if i'm wrong....
it doesnt take that much work, just hold the speaker up and mark where you can drill pilot holes for the screws.
where the hell are you gonna mount 3.5's in the dash?