I am 5'9" about 145lbs and I usually wear a size M. I ordered a L in this coat and it is perfect. There is a little room to layer underneath and it is plenty long (sleeves too). I would definitely call this a 'jacket' and not a 'coat.' It is more than a shell with a little insulation in the jacket, but is not super warm. I love how light it is (compared with a 3-in-1 coat). You can easily stay warm on a cold day if you layer properly or keep cool on a warmer spring day by losing some layers. I just used this jacket to snowboard all day on a 40deg day in the rain and it held up great.