like boogers said, 6x9 in the rear in you have a Sedan. 6 1/2 all around if not. For the front, I got some Alpine type-S and it came with these extenders that screwed into the stock mouting points of the old housing and allowed the bigger magnet to fit. But now my door panel doesnt clip in all the way at the bottom where the speaker is, but its barely noticable. Only when you open the door :hsugh: oh and it is easier if you take off the door panel when installing.
for the rear, you can unscrew the bolts from the trunk and go from there, pretty simple stuff.

should take you a couple hours tops, if even that