Stevevan
It really depends on your needs.
My regular street clothes I wear a large. All my Kuiu is sized large including my guide jacket. If you want the Guide jacket over lots of layers go up a size. My feeling on the Guide jacket, which I like by the way, is it is almost a heavy technical shirt. If you are going to sit in a blind or a stand you will need lots of layers underneath or possibly over it. If traveling not so much. I can wear comfortably underneath my large guide jacket a merino wool (or two if real cold) base layer and a Peloton 200 Hoodie. I used this combo Turkey hunting last spring sitting against a tree in below freezing weather but I started to get a little chilly after about an hour of minimal movement. During my Archery Elk hunting in late September in the high country, which can get pretty cold in the mornings, I wear the same but also carry a down puffy vest and my rain gear top (Chugach - large) which go over the top of the jacket if I am stopped for a length of time and its real cold.
Hope this helps you with your decision.