* The tire sidewall: The tire size is printed on the sidewall of your tire. The rim size is included in this information (e.g., P215/65R15). The "15" in this example indicates a 15-inch rim.
* The vehicle's paperwork: Your owner's manual or other vehicle documentation may specify the rim size.
* The rim itself: The rim size is usually cast into the metal of the rim itself.
Without knowing your specific Malibu's trim level and options, it's impossible to give a definitive answer. The most common sizes were likely in the 15-inch range, but some models might have had 16-inch rims as an option.
