Hi, Annie,
My marmot precip is a women's L -- since I often have a fleece on under it and it's my only jacket on the Camino, I leave space for layers. It's roomy, but doesn't look like I'm swimming in it if I wear it with just a shirt underneath. I'm about 5' 7", 140 pounds, mas o menos.
It's hard to know about sizing. I just bought a couple of ex officio shirts in a M and they are huge on me, so obviously there is a huge variation here.
Hope this helps. Make sure you can return the jacket! Buen camino, Laurie