Most factory speakers back in the early 90's still used paper cones.. which is crap. I don't know the size, but some inexpensive replacements from Good Guys, Best Buy or any place like that would be a vast improvement.
I am guessing the rear's are 4's and the front's either 5 1/4 or 6"
I am guessing the rear's are 4's and the front's either 5 1/4 or 6"