The size of the rim can be read from the markings on it. They consist of a sequence of numbers and letters, for example, 8Jx15H2 ET36, where 8 is the rim width designation given in inches, 15 is the diameter of the rim in inches, and ET 36 is the offset of the rim, that is, the distance of the mounting surface from the rim's geometric center line (specified in millimeters).
On the rim there is also information about the bolt pattern, which can be, for example, 5x115 (5 is the number of mounting holes, and 115 is the diameter of the circle on which they are placed).