Diagrams feature prominently in science education, and there has been an increase in research focusing on students' use of them in knowledge construction. This paper reports on an investigation into first year university students' perceptions of scale and size at the cellular level. It was found that many students appeared to tacitly assume that textbook diagrams presented cellular components in true relative size, leading to widespread interpretative problems with regard to scale and absolute size. The paper includes recommendations for textbook designers and classroom practitioners.