215= 215 MM or 8.41 inches
45 is a series aka a percentage of the width displayed in height from the rim to the tread patch(aka sidewall) or in this case 96.75mm or 3.78 inches.

to get the total over all height you can do rim height+ (2Xsidewall)= total height

again in this case 17+(2x3.78)= 24.56 inches in over all height, ideally you want to maintain as close to a factory circumference to not throw off you handeling and speedo charecteristics.