You can try to use the Analyze particle to automatic measure just open your image in imageJ and change to RGB stack mode. adjust the threshold as precision as possible and than go to analyze and select analyze particle.
dont forget to check list the outline menu and display result in the analyze particle menu and also you can input your object size to exclude the unspecific dot.
The result will show the area, its mean your object size distributions, you can copy this result to excel and analyze statistically this data
Note: attachment file is example in my research to analyze the bacteria counting by staining and imageJ. the unit still in pixel, if you want get the real unit you have to use the measurement figure was taken by same camera and same microscope with your object analysis. and you have to set scale before analysis