normally it goes something like R15 185 65, which is the size Diameter of wheel in inches, then the width in centimeters, then the tyre height, or thickness. The small this last number the lower the profile of the tyre and the more it costs:)
Try and get one size up and one width out from your current wheels or you are in for trouble with your insurance agent, and maybe vicroads. If you do get big ass wheels make sure to let your insurance company know, or they will viod your policy real fast.