If you don’t know your tire size, you can check your owner’s manual, the driver’s side door jamb, and the side of the tires for a series of letters and numbers. The two numbers after R are what you’ll need as they are the rim diameter in inches.
For example, if the standarad BMW 335 I tire size is P225/45R18, the BMW 335 I rim size is 18 inches! You can find replacement rims online or through a shop that speclizies in tires and wheels or customization.