According to Overstock's guide to using a table runner, table runners should be 12 inches longer than the length of the table. At this length, the runner hangs over 6 inches on either side and ideally matches up with the edge of the tablecloth.
A table runner should also be one-third of the width of the table and should be placed directly down the center of the table. If a centerpiece is used, it should be placed in the middle of the table runner. As an alternative, Overstock also suggests placing multiple shorter table runners across the width of the table to create a contemporary look.