This paper presents a design process for the challenging problem of sizing the engine pack for a distributed series hybrid-electric propulsion system of unmanned aerial vehicle. Sizing the propulsion system for hybrid-electric unmanned aerial vehicles is a demanding problem because of the two different categories of propulsion (the engine and the motor), and the electrical system characteristics. Furthermore, what adds to the difficulty is that the internal combustion engine does not directly drive the propellers, but it is connected to an electrical generator and therefore provides electrical power to the electric motors and propellers. Hence there is a clear distinction from the traditional engine solutions which are mechanically coupled to the propeller. This paper addresses this specific distinction and proposes an indirect solution based on properties on the electrical part of the system. In particular, a novel parametric characterisation engine sizing approach is presented using the battery pack state-of-charge during a realistic unmanned aerial vehicle flight scenario. Five candidate engine options were considered with different starting conditions for the electrical system. The results show that by using the state-of-charge properties it is possible to select an appropriate size of engine pack while carrying a suitable electrical propulsion pack. However, the solutions are not unique and are appropriate for given design criteria clearly indicated in the paper.