Use the panel removal tool to remove the studs that are holding the door's panel in place. Once the door panel has been removed, you will see that the speaker is attached to the frame of the car door.
Use the measuring tape to determine the size of the speaker by running it from one side of the cone to the other. Also measure the distances from each of the screws across from each other as well. This measurement will be the size needed for replacing your door speakers.
Reassemble the door panel, placing each stud back into its original place.
Open the trunk of your car. You will see the back portion on the speakers, which will be sufficient for measuring the speaker.
Use the measuring tape to find the distances between the multiple placement screws, and the size of the speaker cone itself. This will give you the size needed to replace the rear speakers as well. Now that you have the proper measurements, you can begin to seriously look at some of the better options that are available to you.