In all cases we recommend you use 2 hangers per single picture or vertical row of pictures and 2 hooks per picture. This will hold the picture straight and keep the whole display neat and tidy.
Small pictures use exactly the same theory as above. In a vertical display, if you use one hanger for a vertical row of small pictures the hanger will act as a pivot for the top pictures caused by the weight of the bottom one pulling away. The vertical row will look untidy.