This bunting is made from a single layer of fleece, so it appears thin, but it is warm without being bulky, so my baby has been very comfortable in it. It's not really waterproof, so I wouldn't have him playing out in the snow in it, but it's warm enough for going on walks or for being out and about in the winter. Also, it's at least somewhat water repellent, as the rain seems to bead off of it (although I've never had my boy out in the rain longer than a quick run from the car to indoors, and I don't think it would keep him dry if he were out in the rain for a long time). Overall, it's a nice, thin, warm bunting for every day use in Boston.
As for sizing, my boy is about 29 inches long and 21lbs. We got him the 18 month size, which fit him well enough that he can use it, but it is still big and roomy enough that he can grow into it and hopefully use it through the rest of the winter.