all four of your speakers are are 6x8s if its a standard coupe. if you have a convertible with the mach system, then your rear speakers are 6.5s. other than that, theyre all 6x8, but you can also fit a 5x7 in there as well.
to remove the panel, you need to just pop off the trim that has the window and door lock switches in it...large plastic one going all the way down to the grab handle. you'll see an indention in the plastic to get a flat head screwdriver...just wrap it with some electrical tape so it doesnt scratch the plastic. there are a couple of phillips head screws behind there.
to the left of the speaker grill on the edge of the panel, there is a plastic push pin. that pulls out...careful, its kind of tough to remove if you dont have a panel popper type of tool....just work it out. (if you have the mach system, you will need to remove the tweeter cover at the top and there is philips screw back there....removing this makes it easier to get the panel off.)
after that just pull the panel up and off....its got these plastic hooks that hold it on there. | 
anyone know how to remove door panel on 2004 mustang? 
