Final answer:
The Mean Corpuscular Volume (MCV) test measures the average size of erythrocytes in a blood sample, important for diagnosing different types of anemia.
Step-by-step explanation:
The blood test that measures the average size of an erythrocyte in a blood sample is the Mean Corpuscular Volume (MCV). This measurement provides insight into the size of red blood cells, categorizing them as normocytic, microcytic, or macrocytic, corresponding to normal-sized, smaller-than-normal, and larger-than-normal cells respectively. Establishing RBC size is crucial part of diagnosing various types of anemia and understanding the overall health of erythrocytes.