This is not an easy question to answer because it will vary from person to person. But, you may be back and forth with the decision of wanting a tight and form-fitting as opposed to loose fitting. Both have their advantages. Possessing a loose fit will allow you to add extra layering of clothes underneath in times of extremely cold climates.
On the opposite side of the spectrum, having a tight fit can be a little more comfortable for people.
Really, this is all about looking out for number one, yourself. If you know you will want excess layering, and possibly a harness, go with a jacket with a roomier fit.