Measure the width of your dining room table. Subtract 12 inches from this number and write down the result. This is the maximum width your dining room light should be; any wider and you risk dwarfing the dining room table. For example, if your dining room table is 50 inches wide, than your dining room light should be no wider than 40 inches.
Measure between the dining room walls and the center of your dining room table (or, if you have an open floor plan, just measure between the single wall and the walkway) to make sure there is at least 48 inches of space to allow people to pass by without bumping their heads on the dining room light fixture.
Measure the ceiling height if you don't already know it and note this on your paper. Your dining room light should hang 30 to 34 inches above the dining room table for 8-foot ceilings, 33 to 37 inches above for 9-foot ceilings and 36 to 40 inches above for 10-foot ceilings.