* The sticker on the driver's side doorjamb: This sticker usually lists the recommended tire size and inflation pressure.
* Your owner's manual: This will specify the recommended tire sizes for your specific vehicle configuration.
* The sidewall of your existing tires: The tire size is printed on the sidewall of the tire itself (e.g., P205/70R15).
Without knowing your specific Lumina model (e.g., Lumina, Lumina APV, Lumina Minivan), it's impossible to give you a definitive answer.