•Total cost-of-ownership oriented sizing of fuel cell heavy-duty trucks.•Optimal fuel cell energy management using dynamic programming.•Assessing economic viability of fuel cell trucks against battery electric trucks.•Fuel cell entails reduction in truck weight, battery pack capacity and retail price.•Right-sizing considering 2020-, 2030- and 2050-oriented cost scenarios. Fuel cell electrified propulsion may currently represent a promising option for long-haul heavy-duty trucks. However, appropriately sizing fuel cell electrified propulsion systems to fully exploit the economic potential of this technology when applied to heavy-duty trucks still represents an open research question. To overcome this drawback, a methodology to size the fuel cell electrified propulsion system for a heavy-duty truck to minimize its total cost of ownership as function of different present- and future-oriented cost scenarios is presented in this paper. Retained cost contributions include both the retail price and the hydrogen and electricity lifetime costs as evaluated by implementing an optimal energy management approach in diversified driving missions. Fuel cell electrified truck powertrain sizing layouts are compared with battery electric powertrain option, and the latter is suggested as more appealing in the present cost scenario. Nevertheless, thanks to reductions forecasted in 10 years and 30 years both in terms of component costs, hydrogen cost and electricity cost, rightsizing the fuel cell electrified truck propulsion system according to the proposed methodology allows demonstrating its economic viability compared with a battery electric powertrain layout.