Up one size will fit perfectly!!!
I had bought a crew neck merino based on my usual fitting (size). That proved to be a disaster for me since I live in a winter climate. Yesterday -40°C. That sweater was ill-fitting - too short for normal shoulders to hip length notwithstanding its softness and superb warmth.
Upsized this turtleneck by one size and.....the arm length and waist overlap was perfect. No cold drafts anywhere on my body. Super warm and comfortable.