Your tyre size is printed on the sidewall, for example 225/45 R17 94Y. Choose the inch diameter above and then refine by width, profile, season and load and speed index. Here 225 is the tyre width in millimetres, 45 the ratio of height to width in percent, R the radial construction, 17 the rim diameter in inches, 94 the load index and Y the speed index.
Not every tyre size is approved for every vehicle. Which dimensions are permitted is stated in your vehicle documents (CoC or registration certificate). For a vehicle-specific selection, use the configurator with vehicle entry.