Complex sentence: The stones are round or oval, of varying sizes and weights, some the size of the tip of your thumb, used between the toes, up to very large stones-2 or 3 pounds in weight.
Split into simpler sentences: The stones are round or oval, of varying sizes and weights. Some stones are the size of the tip of your thumb (these are used between the toes), while others can be very large-2 or 3 pounds in weight.