Among the wide technological opportunities to reduce CO2 emissions in the transportation sector, Waste Heat Recovery (WHR), done by Organic Rankine Cycle (ORC) power units, was subject of a great scienti0c interest. However, several complications apply for an on-board operation and management, which reduce its real benefit in terms of net fuel consumption reduction. Weight increase, encumbrances, engine backpressure, intrinsic off design operating conditions are all negative aspects which should be still solved. In this paper, the interest is focused on the limiting factors of an ORC-based unit operating on board: engine backpressure due to Heat Recovery Vapor Generator (HRVG) presence, surface of the condenser and additional weight. All these aspects behave as limiting factors of the WHR. Considering the mechanical power produced by the unit as a desired input value, the paper discusses the HRVG and condenser sizing as a function of engine backpressure, weight increase and vehicle frontal area required by the condenser. The engine considered was an LVECO F1C turbocharged diesel engine and the reference vehicle was a light duty one. Experimental data have been used for gas temperature and flow rates. The paper considers also the possible presence of an internal regeneration within the ORC which increases plant efficiency (reducing the dimensions of the HRVG and condenser) but introduces an additional heat exchanger. A correlation between weight of the heat exchangers and condenser frontal area as a function of the mechanical power recovered has been presented, without and with the regeneration stage.