Check the result of the Blob tool. Here you can find the number of blobs, individual blob areas and holes. 

Hint: Using the area to calculate the radius will give a high precision results. This is in many circumstances better than using a caliper. The area of a circle is given by : Area = 2*pi*radius. This result may easily be calculated using a PythonScript tool.

A hole is an area inside a blob with greyscale values outside the max and min values set in the blob tool.