Final answer:
The largest difference in anatomical size between adolescents occurs during the adolescent growth spurt, leading to significant sex differences in body composition and shape.
Step-by-step explanation:
The difference in anatomical size between adolescents is greatest during the adolescent growth spurt than at any other time in the human life span. This period involves not only significant increases in height and weight but also brings notable sex differences in body composition and shape. For instance, in males, the shoulders and chest broaden relative to the hips, while in females, the pelvis and hips widen in comparison to the shoulders and chest. Additionally, males may experience continued muscle growth even after their height growth has ceased, whereas females tend to gain more body fat during this stage. These developments lead to the distinct sexual dimorphism observed in adult human physical characteristics.