There are two ways to learn the correct tyre size:
Choosing a suitable tyre size for your car with a few parameters
1. The easiest and most reliable way is to check the size of the tyres you have used before. The picture shows how this information is provided.
WidthHeightDiameterLoad indexSpeed rating
2. Alternative variants of the tyre sizes can be found in the service and repair manual for your car. Furthermore, this information is specified on the pillar, on the front door end at the driver's side, on the fuel tank cap and on the glove box lid. But the best way is to check the size on the old tyres.
or
Be sure to choose a car, otherwise you risk to buy an item that does not fit your car
70 db
SMART CITY-COUPE 
SMART FORTWO 
SMART CABRIO 
SMART FORFOUR 
SMART ROADSTER 
Winter tyres SMART CITY-COUPE (450)