The aerospace industry is currently transitioning to More Electric subsystem architectures due to steadily improving electric technologies and the technology saturation of established conventional architectures. For aircraft with such unconventional architectures, the lack of historical information and the presence of increased inter-subsystem interactions create a significant design challenge. These necessitate a greater focus on subsystems design earlier in the design process than typically seen for aircraft with conventional subsystem architectures. At the same time, however, to be suitable for the early design phases, the subsystem analyses must be computationally inexpensive and not require detailed aircraft definition. This work presents an integrated, modular, and tool-independent approach to the sizing and performance analysis of the aircraft and its subsystems, in which inter-dependencies are established between relevant aircraft and subsystem level parameters. The approach allows the assessment of subsystem architectures using vehicle and mission level metrics for a fixed vehicle design, and also the amplified effect of re-sizing the vehicle in accordance with a pre-defined rule-set. The proposed approach was demonstrated through a comparative assessment of a predominantly electric subsystem architecture and a conventional one for a representative single-aisle aircraft. In this assessment, the impacts of changing the form of secondary power extraction, the change in vehicle empty weight, and re-sizing of the vehicle were successively identified.