ImageJ is an excellent choice as long as you did a camera calibration so you know how many microns per pixel for each objective you took images with. If you know the exact size of any structure, you can use that structure as a calibration reference.

how about getting a calibration microscopic slide?

Carolina Co sells those or a Microscopic Co like Tousimis or with EM?

I also use ImageJ for histology of fish tissues. It is free, open source software that is easy to learn and use.

Thanks everyone for valuable suggestions I will use IMAGE J. jacques Gilloteaux I have used frosted slides. 

Using a diatom from a stream can calibrate with a mm scale with precision for light microscopy, like in a century ago

Dear Sir, jacques gilloteaux, i am not getting your answer. Please could you explain it further.

Cellsens Standart