I wouldn't say the shell is waterproof. I'd imagine water will sort of bead up a little bit on it, but I wouldn't actively wear it on a rainy day. It's a thick/washed twill, 100% cotton I believe. The "black shimmer" version I think may be a different material though? Tough call on sizing for you. I'd say medium but it depends how well distributed your weight is maybe haha. I put a thick sweatshirt under mine and it wasn't crazily tight.