I own 3 of their shirts all 42. I ordered the Glacier shirt in a 42 and it was noticably slimmer. I ordered a 44 to decide which one would be better. The 44 was perfect. The dnim is pretty stretchy so it does give but doesnt remain. My recomendation is to size up one. FairEnds also recommends this.
Also if you intend to wear a s jacket or over anything besides a tshirt. Then may want to go 2 sizes.