Including a familiar object in a scene provides a sense of scale. Choose reference objects that we instinctively know the size of, like people or vehicles.
Most people have the ability to recognize familiar objects and their relative sizes, even if there is a small margin of error. The person by the flag pole gives us a sense of scale allowing us to estimate the height of the lighthouse or the rock it is standing on.
With no recognizable reference, it is difficult to estimate how tall the finger is.
Add some people and the size of the fingers becomes easier to determine.