A drone, or more formally known as an unmanned aerial vehicle (UAV), is a type of aircraft that is not designed to carry humans. Where in their early versions , drones were often controlled remotely by a human operator; they have recently become fully or partially autonomous. Today, there is massive growth in the use of drones in a wide range of applications: military, aerial photography, search and rescue operations, in the field of agriculture, to name a few. The embedded software used in autonomous drones must perform in real-time, allowing the drone to interact with its environment. The Internet of drones (IoD) was born when drones were interconnected via a network allowing them to coordinate. Functional size measurement (FSM), a powerful tool for managing software projects , provides an objective and quantitative base for management decisions. In this paper, we discuss work in progress on COSMIC functional size for estimating energy consumption of drones as an illustration of the potential benefit of using FSM in the context of IoD.