If your "black and white" images are binary images, and you have the Image Processing Toolbox, then you can simply feed them into REGIONPROPS:
stats = regionprops(binary_image, 'Area');
granule_areas = [stats.Area];
This returns a structure, stats, which contains each object's area, which is then turned into a vector, granule_areas. At that point it's up to you to decide how you would like to calculate their distribution.
