I have a black and white images containing different individual granules of different sizes. Is there a code to find the size distribution?. If possible, I would like to calculate the area of the individual particles in the image.
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.