Human red blood cells. Red blood cell distribution width (RDW-CV or RCDW and RDW-SD) is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count.Usually red blood cells are a standard size of about 6-8 μm in diameter.uman red blood cells. Red blood cell distribution width (RDW-CV or RCDW and RDW-SD) is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count. Usually red blood cells are a standard size of about 6-8 μm in diameter.