Hi Miranda,
I have this jacket, and had to size up. The size S was just too snug on my hefty arms and shoulders, especially bent over in cycling position.
The larger size made the jacket's tail very long and a bit loose, and I was worried about it flapping. I did a little seamstressing to get the perfect fit, and it wasn't really hard. I just made a small fold along the seam on either side of the front, from just below the bust widening down to the hem. Taped it down and sewed a straight seam on the inside.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett