How do you determine the ave. grain size based on the SEM image of the sample?

I have a SEM image of my sample and I am to determine the average grain size of the sample.

The grain size and granularity are controlled by reducing the sputter-induced damage in the top layers of HOPG. The sample heating prior to SEM imaging is essential to suppress beam induced contamination. The heating time and temperature are selected to inhibit large increases in the grain sizes of Au particles.