The correct size is 18". I staggered mine with 21" and 19" Bosch Icons. My current set are coming up on 2 years (I got them just after the last Louisville meet after seeing them on another member's ST). The Icons are shaped to hug the windhshield and the spring is internal. Yeah, they'll cost you $20 each, but after you use them, you'll never use anything else, and like I said, I haven't changed mine in almost 2 years, just wipe the edge with a paper towel when I wash the ST.