I have a grand am 03 and have busted speakers in the back.
One way to find the measurements is to go an online forum for that make and model of the vehicle. The best way I found measuring the speakers was to ask people who had done speaker modifications on their specific model what the size was. Another way to do it, is to just take apart the door panel and measure the speaker with measuring tape or a ruler.
A 2003 Pontiac Grand Am has 4x6" speakers in the sides and 6x9" speakers in the rear, just so you know. Hope this helped.
When you remove the busted speaker to fix it yourself, you can measure with a tape measure...if you are having a car stereo company or dealer fix it, no need to worry.