* Your vehicle's sticker: Look for a sticker on the driver's side doorjamb or inside the glove box. This sticker will list the recommended tire size and inflation pressure.
* Your owner's manual: The owner's manual will specify the tire size(s) for your specific Focus model.
* The tire sidewall: The tire size is printed on the sidewall of your existing tires. This will tell you what size was *originally* equipped on your vehicle, although it may have been changed at some point.
Without knowing the specific trim level (e.g., S, SE, SES, ZX3, ZX4, ZX5) it's impossible to give you a definitive answer.