I have the cool wings, but I don't remember choosing a size (I've had these for 7 or 8 years now, so maybe they were onesize back in the day?).

I will say that they sat in a drawer when I lived in NC because I found them to be no cooler than bare arms. Here in Oregon, they work fantastic. I chalk that up to a difference in humidity (we have very little here). I like them mostly for the sun protection but they are definitely cooler than bare arms when the sun beats down. I've actually worn them when gardening (back when we had the farm) and I was out there all day.