In the present world of conventional wind energy, the National Renewable Energy Laboratory (NREL) 5MW reference wind turbine for offshore system development has become an important piece of the puzzle towards a renewable future based on wind energy. DTU showed the benefit of such a reference model with their 10 MW reference wind turbine and continued the evolution in multi-megawatt wind power systems. Even though these two systems exist in the conventional wind turbine industry, a publicly available reference system does not exist in airborne wind energy. Currently, Airborne Wind Energy Systems (AWES) are still in the prototype testing phase as no commercial utility-scale product has been released to the market yet. A reference model like the NREL 5MW turbine can therefore significantly increase the speed of AWES development and open a door towards more publicly available research. This thesis solves the following main research question: "How does a multi-megawatt utility scale airborne wind energy reference system look like, focusing on the main wing parameters?". This is done by setting up a relatively computationally efficient optimisation framework based on a Fluid Structure Interaction (FSI) model combined with a flight dynamics simulation model which can be used in early design optimisations, for example. The FSI model consists of a 3D linear structural Finite Element model coupled with a potential-flow based 3D panel method. The aircraft structure is parametrised and parameters are found for an aircraft with a wing area of approximately 150 m2. The wing mesh and other structural components are created in Matlab while Nastran is exploited to obtain the stiffness, mass and inertia matrices. A model order reduction technique is applied to the structural model, relying on the mode superposition method to decrease the computation effort by several orders. The wing's aerodynamic behaviour is calculated by the 3D panel method. A model order reduction technique is also applied here, based on a Taylor expansion of the aerodynamic influence coefficient matrices. A modified fixed wing aircraft flight controller is used to fly a circular flight path where the ground station periodically allows the tether to be reeled out and in. The navigation of the aircraft is split up in two components, namely the lateral controller (based on a modification to the L1-control logic) and radial dynamics controller (depending on the elevator and tether reel-out behaviour). A Covariance Matrix Adaptation Evolution Strategy (CMAES) optimisation method is applied with a specific objective function to find the system design parameters. This work then presents a detailed representation of the aircraft design, consisting of the planform parameters, material choices and composite layup. It demonstrates the ability of the framework to obtain a wing that can sustain high wing loadings. Also the system performance in a full power cycle illustrates the full potential of a 150 m2 wing which is able to generate multiple megawatts of power. This thesis serves as a foundation of reference systems in airborne wind energy, which other researchers can use and adapt to contribute further to a benchmark network in Airborne Wind Energy.