Each tire size is designed to fit a specific rim width. The correct rim width allows the tire sidewall to flex where the designers meant for the flex to occur. There is a wide range of tires that will fit a specific rim width. After you know what size range of tire will fit your wheels, you can pick a size based on aspect ratio and height of the different tire sizes.
Measure the width of the rim. The measurement is taken just inside the lip or flange of the wheel that holds the tire bead. Wheel width measurements are taken in inches and you should be accurate to the nearest quarter of an inch.
Decide what type of tire in relation to the aspect ratio will go on the wheel. The aspect ratio is the ratio of the height of the tire to the width. If a tire size is 235/60R16, the aspect ratio is 60.