You have the medium chainstay, which is what comes standard. To my eye, looking at just the one picture, your bb is maybe a little bit low, but not, "oh-jeez-get-that-man-a-large-chainstay" low. If you try the large, you might notice a small improvement in overall aerodynamics, but you might also notice a slight worsening of handling at slower speeds and climbing. If you don't climb a lot, it could worth trying. If you try it and don't like it, you can probably find a buyer for it here on the forum Marketplace, with the gap between what you paid and how it sells making it more like a rental fee. I use the small chainstay, and if I remember right, I paid about $250.00 from CB a few years back.