A microscope consists of an objective lens having a focal length of 16.22 mm and an eyepiece having a focal length of 9.40 mm. With the length of the barrel set at 28.0 cm, the diameter of a red blood cells image subtends an angle of around 1.43 mrad with the eye. When the final image distance is around 28.0 cm from the eyepiece, determine the actual diameter of the red blood cell?