The size of the lamp is dictated by the size of the room of course. If the room is 10' x 12', the minimum diameter of your fixture is 22".
Diameter = Room length + Room width
Try your best to preserve the proportions of the room. One of the joys of living in a brownstone is having high ceilings. I like to mount my curtain rods halfway between the bottom of the crown molding and the top of the window frame. Extra long curtains that puddle on the floor are formal and romantic. But, I usually hem mine one inch off the floor so they don't get dirty.