You will find the size of a tyre on its sidewall.
In most cases a tyre’s size will be displayed as follows:
Width | Profile | Rim Size | Load Rating | Speed Rating
with a typical size for a 4x4 tyre being
255 / 65 R16 (109) H
It is worth noting that a 4x4 tyre’s sidewall will differ slightly from a car tyre. On many 4x4 tyres you will find an LT before the tyre size. This stands for light truck and lets you know that the tyre had been constructed to be reinforced and can deal with the extra weight of a 4x4 tyre.