This paper clarifies the terminologies used to describe the size effect on fatigue behaviour of welded joints. It summarizes the existing research on size effect in the perspective of newly defined terminologies. It identifies knowledge gaps in designing tubular joints using the hot spot stress method, i.e. thin-walled tubular joints with wall thickness less than 4 mm and thick-walled tubular joints with wall thickness larger than 50 mm or diameter to thickness ratio less than 24. It is the thin-walled tubular joints that are addressed in this paper. It is found out that thin-walled tube-to-plate T-joints do not follow the conventional trend: the thinner the section is, the higher the fatigue life. It is also found out that simple extrapolation of existing fatigue design curves may result in unsafe design of thin-walled tube-to-tube T-joints. The effect of chord stiffness on the fatigue behaviour of thin-walled tubular T-joints is also discussed. KEYWORDS: Size effect, Thickness effect; welded joints; plate; tube; weld defects; fatigue.