This algoritm provided by PyImageSearch measure the objects in the image based on the left-most object.
Peruvian sol dimensions
Trout example dimensiones
git clone https://github.com/DZPeru/fish-classifier
pip3 install -r requirements.txt
python3 measure.py --image image.jpg --width 2.5
Where width is the width of the left-most object in the image in centimeters. In the case of peruvian sol (PEN), the diameter is 2.55 cm.