Numerous researches about hybrid electrical vehicles (HEVs) deal with topologies, technologies, sizing and control. These aspects allow reducing transportation costs and environmental impacts. This study focuses on the sizing of the electrical machine (EM) of the HEV, taking into account its surroundings: the hybrid system, the driving cycle and an optimal energy management. In this study, the parallel HEV is the study case. In a classical HEV design process, a scaling factor is usually applied on an efficiency map model to fix the standard power of the EM. The efficiency and the maximum torque power are scaled using a linear dependency on the rated maximum power. However, this method has some disadvantages. This study proposes two formulations of a scaling model based on a magnetic circuit model (MCM) with one or ten parameters. Then, the MCM is involved in a multi-objective optimisation process of the HEV. This process is a global sizing process using dynamic programming as an optimal energy management. Optimal sizings of the hybrid vehicle are then proposed for various driving conditions.