A medium Icebreaker t shirt fits me nicely....
A medium Icebreaker t shirt fits me nicely. Should I size up to a large in this to allow for layering?
Depends on how you want it fit. The RealFleece run true to size and are not a tight fit. If you want it bigger then size up. You are also going to find this offers a lot of warmth and you won't need to wear a lot underneath. Hope that helps!