The sidewall of a tyre shows a sequence of characters being the tyre size, e.g. 225/60 R17. It is very important to decode it before the purchase. 225 is the cross section width measured between the tyre sides in millimetres.
The second number in tyre size is the height of profile (aspect ratio), i.e. a percentage to tyre width, e.g. 60 is 60% of 225, i.e. 135 mm. ”R” indicates the tyre type (R - radial, D - diagonal).