To choose the right tyre size for your car, check the car manufacturer's specifications. Refer to your vehicle's handbook for approved sizes. You can also use the size of the tyres you are currently using, but make sure they are the right size for your car (especially if you are buying a used car and this is your first tyre change).
Note that it is not only the size of the tyres that is crucial, but also the load and speed indices, indicating the relationship between the maximum load allowed and the maximum speed.