To find the exact size, you'd need to:
* Consult your owner's manual: This is the best source. It might specify the exhaust system's dimensions.
* Check the exhaust system itself: Look at the pipes. They're usually stamped with a size designation (in inches or millimeters). This requires getting under the car.
* Use a parts catalog: Online auto parts websites (like AutoZone, Advance Auto Parts, etc.) allow you to look up parts for your specific vehicle (year, make, model, and engine). Searching for "exhaust pipe" or "exhaust system" will show available replacements, and the specifications will likely be listed.
Without more specific information about your Malibu's engine (e.g., 2.4L, 3.1L, etc.), it's impossible to give a definitive answer.