In order to determine the right size chandelier for a room you must take into consideration not only the size of the room but also the furnishings in the room, the overall decor, and the "visual weight" of the chandelier.
A general rule for determining the optimum diameter of a chandelier is that the sum of the room's length and its width in feet will be equal to the diameter of the chandelier in inches. Accordingly, a typical 11' by 14' dining room will usually want a chandelier that is about 25 inches across. Because of its visual weight, a dense crystal chandelier 20 inches in diameter may work and by the same criterion, an airier chandelier 30 inches in diameter, with delicate branches and small lights, may work if it's being placed over a round table.
An exception to this rule is the case of a long, narrow room, in which case the chandelier should have about two inches of diameter for every foot of the room's width. For example, a dining room 9' by 16' would demand a long narrow table and likely two chandeliers, about 18" across, both placed over the long table.
But the boldest decorating statements almost always break the rules. If you were to place a 48" round table at one end of that same long, narrow room and surround it with heavy upholstered chairs, you could ignore the above rules and instead apply the one that says the diameter of a chandelier over a table should be at least 12 inches less than the width of the table and no less than half the diameter of the table. That would allow us to consider chandeliers of anywhere between 24" and 36" across. The decor and how you plan to use the room will be the determining factors as much as the size of the room.
A small 9' by 9' bedroom or a hallway won't ordinarily handle a chandelier, but increase the volume with a vaulted ceiling, and you'll want a substantial fixture to fill the vertical space. Ordinarily, the bottom of a chandelier should be seven feet or more above the floor. Over a bed you'll want more space for flicking the sheets, and over a dining table you can ignore that rule and instead hang your chandelier so that the bottom is 30" to 36" above the table top. Common sense rules here: if people will be waking under the chandelier it has to be higher than if it's lighting the dining table.
Choosing the right chandelier and hanging it correctly can transform a room from ordinary to dramatic. Part of getting it right is to keep in mind that size does matter.