Body size estimation errors are common but are specifically exaggerated in individuals with eating disorders. The cause of these biased estimates is poorly understood. This thesis examined the perceptual factors that contribute to body size misperception, finding that serial dependence and regression to the mean biases, separately contribute to estimation errors. Adding context to these findings, biases were larger for synthetic, and moving bodies. Of clinical interest, serial dependence bias was associated with eating disorder symptoms, in young women. This thesis advances our understanding of the factors that contribute to body size misperception in common observers and eating disorder populations.