How should a thermal jersey fit compared to a summer jersey?

A thermal jersey should fit close to the body like a summer jersey, but with slightly more room to accommodate a base layer underneath. The key difference is that any gap between the fabric and your skin becomes a cold spot in winter, so the fit needs to be snug. However, don't size down — if anything, size up slightly if you plan to wear thicker base layers. The jersey should seal at the wrists, neck, and waist without being restrictive. Test the fit with your intended base layer on.