The light unit should be close to the water surface (for an aquarium, a terrarium is different). The higher the light, the less it will penetrate the water. This is important if there are plants. As for lighting the whole aquarium, most lighting units are linear and intended to sit end to end, somewhere around the middle front to back or slightly back of this.
Another issue is that the higher the light source is above the tank frame, the more difficult it is to enjoy the aquarium because you have light shining in your eyes, unless you build some sort of box enclosure. Having the light unit across the frame end to end usually works very well.