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.
If you want to hang two chandeliers over an exceptionally long dining room table (for example a table that is approximately 54" wide x 120" long), then if you select chandeliers that are 1/3 the width of the table and hang them centered on each end of the table that would be correct. Therefore, each chandelier would be approximately 18" in diameter.
The height of the actual chandelier should be determined on the ceiling height of the room.
Rule of Thumb: allowing 2 1/2" - 3" for each foot of a room's height.
Using this calculation, a room that has an 8 foot 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.
The chandelier should be hung centered over the table (unless you are installing more than 1 fixture over the table) and the bottom of the light fixture should be about 30" above the table top.