First off, I’ll provide a guideline: The bottom of the fixture should be 32 to 36 inches above the tabletop. Now that I’ve told you that rule, I’m also going to tell you to disregard it. I really feel that every situation is different. It depends on the size of the table, the diameter and height of the fixture, and the height of the ceiling. All of these are factors in determining what should be the correct mounting height.
Most lighting manufacturers offer additional cord, chain or stem lengths to provide some give-and-take in the overall length of the fixture. I like to have my client sit down in a dining chair while the electrician is there so that we can look at various heights.
I stand at the entry of the dining room to see how the fixture relates to the volume the space.
Also, when picking a fixture (if you can’t borrow an actual sample), consider cutting out a cardboard template that is the height and diameter of the fixture to give a good idea of how it might look in the space before making a final decision. You may find that two medium fixtures look better than one large fixture, especially with a rectangular table.