Accurate arm measurement is difficult, but it's only an issue when length is uncommonly long or short, and then only when deciding if you need a hardcore endurance geometry. You can decide that just by looking in a full-length mirror. For most of us, swapping and adjusting the stem is all we need to accommodate arm length. Your leg length is about average for your height and the standover is about right. You could straddle a Medium but it wouldn't be fun. Get the Small.
I test rode a slightly older version of this bike and the handling was phenomenal, especially after a morning of messing around on endurance bikes.