Knowing the "sizes" is the first thing to check to buy the right tyre. This information refers to the width, the height (or technical series) and diameter of the rim and are expressed with the typical acronym 215/55 R17.
(see the image "tyre legend")
The simplest thing to do is to write down the measurements provided on the tyres already mounted on your car. Tyres with difference measurements can be mounted as long as the manufacturer of the vehicle has considered this possibility by type-approving other sizes and including them on the vehicle registration document.
Ultimately, tyres with non-conforming sizes with respect to the manufacturer's instructions cannot be mounted. Moreover, we must take into account that it is obvious that tyres with a different rim size as to the one already being used can be mounted, for example from R15 to R17 (as long as this is provided in the vehicle registration document) and the rims would have to be replaced with rims of the same diameter.
Moreover, all tyres mounted on a car (expect for the spare tyre) must have the same size and those on the same axle must be the same.