Below is an image (graciously provided by R. Anggraini) that illustrates the general procedure for analyzing spot sizes. This image is very difficult to analyze because from the computer's point of view, there are actually two different types of spots: the white reflections and the dark drops. There are also some light blobs that are less visible.
The normal procedure for counting grains would only measure the size of the reflections on the drops when in fact, the reflections are only incidentally related to the drop size. To analyze this image, it's necessary to filter the image to make the drops black and the background white, as much as possible, before trying to measure the sizes. Here is a typical procedure:
Now the drops are black spots on a gray background. We need to enhance the edges and change the spot color to black.
After counting, the image looks like this. The program draws rectangles around the image to indicate the minimum and maximum x and y values. (This is for convenience only; the actual measurements are performed on a subset of pixels within the box.)
The spot outlines have been changed to red for easier visibility.
This counts most of the drops, although a few are split in half. It could be improved by filtering with other filters, such as the Maximize Local Contrast filter. If all else fails, it may be necessary to draw a line manually between drops if they are overlapping, or to manually paint out the white reflections. Anyway, this is the general procedure: filter the image until the spot is a solid black blob on a uniform background, then count the grains.