A tyre has a lot of information written on the side wall of the tyre, that is, the part of the tyre that is not in contact with the road. The side wall has info like tyre size, manufacturing date, speed rating, manufacturing place, company name, model of the tyre.
To know the tyre size of your car, you should know the format in which the tyre size is written, for example. 195/55R16. 195 denotes the width of the tyre, 55 denotes the profile of the tyre and 16 denotes the inches. All tyres have the size written on them in this format regardless the tyre company.
You might get a clear understanding buy looking at this picture below –