Taken from the link below:
One universally correct formula for determining the correct size light fixture for the Dining Room is to go with a fixture that has a diameter that is half the diameter (or width) of the tabletop. For instance, if the Dining Room table is a rectangle of 48" wide x 72" long, a chandelier that is approximately 24" (half of the 48" width of the table) in diameter would look fabulous. If the table is a 60" round diameter, a 30" diameter chandelier would be appropriate.

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The height of the actual chandelier should be determined on the ceiling height of the room. Many Designers follow the rule of allowing 2 " 3" for each foot of a rooms height. Using this calculation, a room that has an 8' ceiling can use a chandelier that is approximately 20" to 24" in height. If the ceiling is 10 feet high, than a light fixture that is 25" to 30" would look more appropriate.

Sue

Here is a link that might be useful: How to Pick the Best Size Chandelier for your Room

Chemocurl thanks for the link

Great info chemocurl! Thanks