To determine the optimal size of the tires you need, you have two options:
You can read the tire size on the sidewall label on the tire to be replaced. Please note that there may be different information between summer and winter tires.
In the registration certificate introduced in 2005, the tire dimensions are listed under points 15.1 and 15.2. However, this does not have to match the actual mounted tire size. Alternative tire dimensions are permitted in many cases. In old vehicle certificates the tire sizes can be found under 2.1 and 2.2.