In main menu--Settings--you need to check the box for 'Unrealistic Wheels.' This will enable you to select higher / lower tire size values when buying them. I don't subscribe to that mod though so, I'm not sure what their 'default' sizes were on that car. However, if you go into your PC's folders, down inside the steam folder, to the \Workshop\Content\.... folder for this game, you can find the car in question and open it's 'config.txt' file. Check in the section about wheels and it should list the tire size that is default.