The importance of "size and scale" in nanoscience and engineering has been recognized by both scientists and science educators. A solid understanding of this concept is key to the learning of nano-related materials. However, little research has been done to explore how students understand this crucial concept in the undergraduate context, and the existing few studies report much difficulty students experience in learning it. In this talk, we will present a series of studies that are aimed at unpacking student conceptions of "size and scale" in the context of undergraduate nanoscience/engineering courses. Informed by Variation Theory and the Construct-Centered Design, we revealed a preliminary typology describing the range of conceptions our students demonstrated, from which we subsequently constructed an instructional intervention that was shown as successful in helping students gain a more sophisticated understanding of "size and scale".