Measure the height of the ceiling in your room. Keep in mind, higher ceilings usually require larger fixtures while lower ceilings require smaller fixtures. A common favorite is to 2.5 to 3 inches for every foot of height for a room. So if the ceiling is 10 feet high, the fixture should have a height of between 25 and 30 inches.
Determine the square footage of your room. Let's assume the room is 15 by 20 feet. Add up the width and length you calculated, in this case 15 + 20 = 35. The perfect diameter of the chandelier for a room like this would be approximately 35 inches. These are all approximative dimensions.
Consider the "visual weight" of the chandelier as different styles can make a chandelier appear larger or smaller than they actually are.