* Check the sticker on the driver's side doorjamb: This sticker will list the recommended tire size and inflation pressure. 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 of your current tires (e.g., P225/60R17). However, this only tells you what's currently on the car, not necessarily what's *recommended*.
Without one of these sources, I cannot provide the correct tire size.