I am fairly thin but tall so thought a medium would be a good fit for me.  When I tried it on, I was swimming inside the jacket and the length was even a little long for me.  I normally wear a 39 regular sport coat and 33 shirt length, to give you an idea of size.  I think a small would have been a better fit for me but I just returned it rather than try the small.  Overall the jacket was good quality, very warm, and looked really good.  Just too big.