The design and the supervision of hybrid electric vehicles (HEV) are strongly coupled. The mutual influence between the optimal components sizing and the optimal operating points choice makes the problem complex. This was previously exposed in literature for spark ignition (SI) HEV. In this paper, we address the same issue for diesel HEV. In this case, the energy management strategy must take nitrogen oxides (NOx) emissions into account in addition to fuel consumption. This paper presents an optimal supervision strategy and its impact on the electric components sizing. The energy management strategy is based on the equivalent consumption minimization strategy (ECMS) using Pontryagin's minimum principle. It allows an adjustable trade-off between NOx and fuel consumption to be minimized. It was validated experimentally with a hardware-in-the-loop test bed. By moving the engine operating points into optimal zones, this strategy allows important NOxemissions reduction while keeping interesting fuel consumption. In comparison to a fuel consumption reduction-oriented strategy, the integration of NOx emissions deeply changes the energy management strategy choices and, as a consequence, the optimal electric components size. Indeed, to limit NOx emissions engine high-load-operating points are avoided and a smaller hybridization level is sufficient. Compared to the SI engine, the diesel has a better efficiency at part load. As a consequence, high NOx reduction is possible without important fuel consumption increase. The interactions between electric components sizing and energy management strategy design are studied in this paper. Depending on the chosen value of the adjustable trade-off of the strategy, the optimal hybridization level changes because the engine optimal operation areas move. The driving cycle influence is also studied here to draw global conclusions.