* The sticker on your driver's side doorjamb: This sticker lists the recommended tire size and inflation pressure for your specific vehicle.
* Your owner's manual: This will also specify the recommended tire sizes.
* The sidewall of your existing tires: The size is printed on the sidewall of each tire. This is a good starting point, but it's always best to confirm with the doorjamb sticker or owner's manual.
Without knowing the year and model of your Cavalier, it's impossible to give you the correct tire size.