Being practical, the extremes are bad. It should be possible to hit the ceiling with a normal fly swatter. It should be possible to stand on a normal chair to

Being practical, the extremes are bad. It should be possible to hit the ceiling with a normal fly swatter. It should be possible to stand on a normal chair to change a lightbulb. As long as those requirements are met, taller is better. Height minimizes the risk of impact with light fixtures. That comes out to be 9 to 10.5 feet. Note that staircases are special. People do not merely stand on each step. People leap up and down the stairs, so extra height is needed to reduce the risk of impact and a serious injury. The ceiling right at the bottom of the stairs, on the landing, is commonly far too short.