The place where you buy the speakers should have a catalog that shows the
size of the speakers. You probably should double check the rears too, they
could be 4x10 for example.
To remove the seat first push down and back on the front of the bottom seat.
Its not always easy and you have to get tough with it, that will unhook it
(one hook on the left, and one on the right) and it will pull right out.
then you will see bolts that hold the back of the seat on. From there find
everything that holds down the rear deck panel and remove it. Its not really
that tough of a job, just dig in and go for it.