* Check the sticker on your driver's side doorjamb. This sticker lists the recommended tire size and inflation pressure for your specific vehicle. This is the most reliable source.
* Check your owner's manual. The owner's manual will also specify the recommended tire sizes.
* Look at the sidewall of your existing tires. The size is printed on the sidewall, usually in a format like P215/65R15 or similar. This will tell you the size currently fitted, but it might not be the only size that fits or is recommended.
Without knowing the specific trim level of your 2005 Malibu, it's impossible to give you a definitive answer.