I'm 5'-9", 170 athletic build. I have a medium and a large and they both work for me. The medium fits close, but not tight. There's room for a thin thermal top (brynje mesh) and tshirt underneath without restriction. This is my go-to layering for cross country skiing. It's perfect sweat control in the cold. The large fits loose without looking sloppy. Perfect for hanging around the house and running errands. Both kind of amaze me with the temperature range. Not good at blocking wind, but they breathe well. Both are excellent at keeping me warm when it's cold and comfortable when exerting or when the temps start rising. I wear both year-round in Central Oregon, east of the cascades. A year into the medium with lots of use and it still looks good, if a bit "patinaed". No piling. They dry remarkably quickly after being wet. Really great and highly recommended. If I were to pick one size for my build, I'd go with medium.