i would say an 8 is the average size of brush to start off with. as you gain more experience and control of your product you may want to go up to a larger brush. though some prefer to stay at 8. i have also noticed that sizes do vary a little between brands, one companies 8 being more like anothers 10.
the ferrules are normaly round , oval or flat and this gives a different shape to the brush head because of the way it holds or pinches the hairs into place.
i dont particularly care for a round brush, though i did work for a long time with a flat brush. i have for the last few years stuck with an oval brush with a pointed tip.
it does seem to be something that varies with different peoples individual preferences and styles of working.