You havn't mentioned the model of the car therefore it won't be possible to say what tyre size is suitable for you car. Some premium car do use different size tyres in the front and rear. Please see the OEM tyre size in your owner's manual and crosscheck with that. If from factory your car comes with same tyre size on all four wheels then I will suggest you to do that with your car.
Differently sized tyres on a vehicle can be dangerous as it can upset the suspension setup and balance of the vehicle.