Nice vest. For fit: I am 5'5", 125-130 lbs. I am generally a small in tops and jackets. I got this in an xs and it fits.  I have a Columbia omni-heat jacket that I really like, but Columbia items are big around the body for me. I really like the omni-heat items (I have some boots too).  Like the jacket, this vest feels light-weight when worn, which makes it very comfortable.  It's just weightless insulation.  The omni-heat makes it surprisingly warm - at times.  These items are great for cold weather active wear, since they retain your body heat like space blankets.  If you are producing any extra body heat, they are like toaster ovens, but they allow your sweat to evaporate better than some heavier items.  If you are just sitting around outside, they're not very warm - at least for me, but I tend to be cold, and by body temp is usually closer to 96 than 98.6.  This is a great layer for cold weather trail runs, with just a base layer underneath, or a shell over the top. The silky fabric keeps it from tugging, and it's not bulky.  It's a nice addition in my office or home in the winter too, when I don't want another sweater over my sweater.  It can definitely up the toast factor.  My only complaint is that it doesn't have a chest pocket, internal or external.  I wish this was standard for outdoor gear.  If I'm on a run or even a hike, I love to be able to stash items within easy reach, without having to take my camelback off to get inside of it.  A chest pocket is the perfect place for my phone, where it's safer than in a back pants pocket where I could fall on it, so I can take a quick picture and keep going.  Or to keep chapstick.