* Check your owner's manual: This is the most reliable source. The manual will specify the correct wiper blade sizes for your specific model year.
* Look at the current wipers: The size is usually printed on the wiper blade itself.
* Use an online resource: Several websites (like AutoZone, Advance Auto Parts, etc.) have lookup tools. You'll need to enter the year, make, and model of your Grand Am GT to find the correct sizes.
Without knowing the year of your Pontiac Grand Am GT, I cannot provide the correct wiper blade sizes.