To find your tyre size you can look at the numbers on the side of your tyres.
You should check the size on the front tyres and rear tyres. Sometimes they differ. For example some Porsche and BMW cars often run wider tyres on the rear for more traction.
If you have a secondhand vehicle it's best to check the tyre placard for the correct size as an alternative or incorrect tyre size may be fitted.
If your car is fitted with incorrect tyres it may not be covered by your insurance or your car may be defected if it is inspected by police.
Using the illustration above:
215 is the tyre width in millimeters.
It's measured across the tread from sidewall to sidewall.
45 is the tyre series or profile.
It's the height of the sidewall measured as a percentage or the width. Basically it's 45% of the tyre width which is 215mm. Nobody ever makes this calculation but the sidewall height would be about 96.7 mm.
17 is the wheel diameter in inches.
Wheels are measured in inches. So this tyre suits a wheel that's 17 inches in diameter.
91 is the load rating.
All you need to know is that the minimum load rating is specified by the manufacturer on the tyre placard and you can't choose a tyre with a lesser rating.
The load rating means nothing without access to a load index table. The index is a numerical code associated with the maximum load a tyre can carry at the speed indicated by the speed symbol. A tyre with a 91 load rating is capable of carrying 615 kilos. Having 4 tyres the total load carried by the vehicle including it's own weight would be 2,460 kilos if the weight was evenly distributed.
W is the speed category.
As with the load rating the manufacturer sets a minimum load rating alphabetically and you can't choose a tyre to the left of it on the alphabet. A W rated tyre can carry the load specified on the load rating up to 270 km/h. If you were to choose a V rated tyre it can only carry the load up to 240 km/h and would be too low.