I ran Pixcavator with the image. Its quality is uneven, so it's hard to capture each crystallite individually. Therefore measuring is almost impossible. Best you can do is to count the cells. The analysis shows that there are 359 of them. Then the average size is 266*175/359 = 130 pixels. Some information about the shapes can be found in a similar fashion. Higher resolution will give better accuracy. Compare to Corneal endothelial cells of the human eye.
Other image analysis examples