Final answer:
The mean corpuscle volume (MCV) is the RBC index that measures the average size of red blood cells and is crucial for diagnosing various types of anemia.
Step-by-step explanation:
The RBC index that indicates cell size is mean corpuscle volume (MCV). MCV measures the average size of red blood cells, also known as erythrocytes. Cells with a normal size are classified as normocytic, those smaller than normal as microcytic, and larger than normal as macrocytic. These measurements are important in diagnosing different types of anemia caused by factors such as blood loss, faulty RBC production, or excessive destruction of RBCs.