Should I size up or down for an endurance jersey?

Size to your normal measurements first. The endurance fit is designed to be close but not constricting. If you are between sizes, size up rather than down. The jersey should feel snug but allow full chest expansion for breathing. A jersey that is too tight will restrict breathing on climbs and cause discomfort on long rides. Remember that the jersey should fit correctly in your riding position, not standing upright.