The smallest is the size of small collar button (good for picking out pin-sized objects), the largest is the size of a box of matches (good for picking out bangle-sized objects). THE PICTURES ARE NOT TO SCALE!
These are made of Neodymium, incredibly powerful, in fact, so powerful that if you get two of them joined together you might not be able to pull them apart. Great care must be taken not to get fingers trapped between the magnet and sheets of metal. Use these to 'skim' large quantities of scrap and pick out anything magnetic in fractions of a second.