Issues related to environmental concern and fossil fuel exhaustion has made wind energy the most widely accepted renewable energy resource. However, there are still several challenges to be solved such as the integrated design of wind turbines, aeroelastic response and stability prediction, grid integration, offshore resource assessment and scaling related problems. While analyzing the market of wind turbines to find the direction of the future developments, one can see a continuous upscaling of wind turbines. Upscaling is performed to harness a larger resource and benefit from economy of scale. This will pose several fundamental implications that have to be identified and tackled in advance. This research focuses on investigating the technical and economical feasibility and limits of large scale offshore wind turbines using the current dominant concept, i.e. a three-bladed, upwind, variable speed, pitch regulated wind turbine installed on a monopile in an offshore wind farm. Thus, the objective of this research is to investigate how upscaling influences the offshore wind turbines. Specifically, following questions are of interest: 1. How do the technical characteristics of the larger scales change with size and can these technical characteristics appear as a barrier? 2. How does the economy of the future offshore wind turbines change with size? 3. What are the considerations and required changes for future offshore wind turbines? To address these questions, a more sophisticated method than the classical upscaling method should be employed. This method should provide the detailed technical and economical data at larger scales and address all the design drivers of such big machines to identify the associated problems. However, interdisciplinary interactions among structure, aerodynamics and control subject to constraints on fatigue, stresses, deflections and frequencies as well as considerations on aeroelastic instability make the development of such a method a cumbersome and complex task. Among many different methods, integrated aeroservoelastic design optimization is found to be the best approach. Therefore, the scaling study of this research is formulated as an multidisciplinary design optimization problem. This method enables the design of the future offshore wind turbines at the required level of details that is needed to investigate the effect of size on technical and economical characteristics at larger scales. Using this method, 5, 10 and 20 MW wind turbines are designed and optimized, including the most relevant design constraints and levelized cost of energy as the objective function. In addition to the design of these wind turbines, the method itself shows a clear way forward for the future offshore wind turbine design methodology development. Based on these optimized wind turbines, scaling trends are constructed to investigate the behavior of a wind turbine as it scales with size. These trends are formulated as a function of rotor diameter to properly reflect the scale. Loading, mass, cost and some other useful trends are extracted to investigate the scaling phenomenon. Blades and tower as the most flexible load carrying components are examined with more attention. Using these results, the challenges of very large scale offshore wind turbines up to 20 MW range are explored and identified. These results demonstrate that a 20 MW design is technically feasible though economically not attractive. Therefore, upscaling of the current wind turbine configurations seems to be an inappropriate approach for larger offshore wind turbines.