Notes:
The find objects operation will extract the features we asked for. It will measure the size of all objects and calculate the average.
We restrict the objects to be taken into account by their size.
There are 206 plaques in the image. Their average size is 211 square pixel.
The find objects operation creates a mask of the objects found and an outlines image of them.
By merging the outlines image with the input image we can control if the detection of the plaques worked as expected.