The general rule of thumb when deciding what size fixture is right for your room is to take the dimensions of the room in feet, add them together and buy a chandelier that many inches in diameter. A 12-by-15 room, for example, would need a 27-inch fixture.
If you're going to break that rule, the dining room is the place to do it. Because the lighting is usually the main focal point of the dining room, a larger chandelier makes a nice statement.