Brent wrote an answer about Outdoor Research Foray Jacket - Men's on March 19, 2012
I'm 5'8" and 150 currently, usually 140/145 and I DO have this jacket and and OR Mentor. The Mentor is a med, a little baggy but makes a good winter shell since its pro-shell. The Foray I have is a small and fits even better. Its a little hard to layer over a thick puffy but for a fleece or say nano puff its great. You also need to consider where you like your sleeves to stop. With the medium I can pull my hands up in the sleeves and keep them dry in the rain, but the small stops just past my wrist. Also, with a medium you have more coverage down past your waist. Those are things to consider but I recommend the small.