Which artistic convention identifies narmer as a king during the early dynastic period?

It is the Hierarchic Scale. Various leveled extent is a procedure utilized as a part of the workmanship, for the most part in model and painting, in which the craftsman utilizes unnatural extent or scale to delineate the relative significance of the figures in the fine art. 
The hieratic scale is a framework used to outwardly convey control in Egyptian and in addition the specialty of different societies, including the antiquated Near East and in medieval European workmanship, for instance. Noteworthy or critical people, for example, pharaohs, were delineated as being substantially bigger than any figures in a scene.