Michael J.wrote a review of Arc'teryx Zeta LT Jacket - Men's on July 20, 2018
Familiarity: I've used it several times
Fit: True to size
I had to return the medium for a large. If you're using the jacket in warm temperatures with a light shirt layer underneath you could get away with medium. However, I use the rain gear in cooler climates where I have a warm under layer - example Iceland I need to throw the jacket on at a waterfall or chilly rain and wind.
I think it runs true to size. If you need it as a "shell" for colder weather get one size up.