Solid Oxide Fuel Cell (SOFC) systems for aviation applications have been considered for their use as APUs or powertrains, as standalone systems or in combination with gas turbines, and both fueled by reformed Jet-A or by hydrogen. Most existing studies focus on powertrain performance modelling. In this work, traditional tube-and-wing regional aircraft sizing methodologies are modified to account for liquid hydrogen fuel and SOFC-GT-Battery hybrid electric powertrains. Methodologies for powertrain modelling, power sizing, energy sizing, weight calculation and system integration have been derived, implemented, verified, validated when possible, and used to assess several study cases. Considering state-of-the-art metal-supported planar ITSOFC technology installed in a 50-seat regional aircraft as test case, it is concluded that current SOFC system power density is not sufficient to achieve MTOW values similar to conventional kerosene-powered regional aircraft. This work serves as a baseline for the integration of sizing methodologies for SOFC-GT-Battery powertrains into aircraft conceptual-to-preliminary design tools.