The Tyre size is embossed into the side wall of the Tyre. Some vehicles have different Tyre sizes front to rear so make sure you select the right size. The picture below illustrates what you are looking for.
Car Tyre sizes are made up of a series of different numbers and letters.
For example the Tyre illustrated is 225/45 R17 93W XL. This is made up of the following information: