In the wake of growing concerns over petroleum supplies and air pollution, hybrid electric vehicles are being developed as safe alternatives to conventional engine vehicles. While the representativeness of standard driving cycles may be questioned, the vehicle electrification brings up new design issues in terms of power rating for energy savings purposes. The present paper aims at proposing a methodological approach for assessing a suitable power rating of the requisite energy sources for the vehicle propelling. Based on a Markov matrix formulation, a vehicle mission modeling technique is proposed, stochastic mission profiles are generated and the dominant one is identified. Then a sizing algorithm is implemented in order to minimize the fuel consumption on that most constraining mission. The application on several kinds of vehicle mission natures will then be proposed and discussed so as to show their impact on the traction chain sizing.