I purchased the Large but I usually take a medium. I'm 5'10" 160lbs with a medium build and a 15 1/2" neck
The large is a little long in the arms and the neck is a little loose but it will be fine with a t-shirt underneath for winter hiking. I agree, I would consider this a midweight shirt.
This review was written in the old system and had content requirements that are different than reviews written today.