Measure the width of your dining room table and write it down on a piece of paper. Then, subtract 12 inches from this number and circle it -- your dining room light should be no wider than this number. For example, if your dining room table is 48 inches wide, then your dining room light should be no wider than 36 inches.
Measure the distance between the center of the dining room table and the dining room walls (or the single wall if you have an open floor plan) to make sure you'll have at least 48 inches of clearance between the dining room light and the walls to maintain proportion.
Measure the distance from the floor to the ceiling if you don't already know your ceiling height (or aren't sure). Write this down on your paper. Your dining room light needs to hang between 30 and 34 inches above your dining room table if your ceilings are 8 feet tall, 33 to 37 inches for a 9-foot ceiling, and an additional 3 inches for every foot taller your ceiling is.