To date, space launchers remain prohibitively expensive and the low reliability compared to commercial aircraft stands out as the main obstacles of future space exploration plans. To address these challenges, future launch vehicles necessitate a paradigm-shift towards more reusable and sustainable assets. In the current fast evolving landscape of competitive launch vehicle design, the development of a dedicated Reusable Launch Vehicle (RLV) design methodology becomes crucial. This thesis outlines the development steps towards an innovative sizing methodology for Single Stage to Orbit (SSTO) with Horizontal Takeoff and Horizontal Landing (HTOL) capabilities. Beginning with the elicitation of mission requirements, constraints, and assumptions, the methodology encompasses considerations such as vehicle geometry, propulsion strategy, and technological availability, thus defining conceptual configuration alternatives complemented by initial estimates of dimensions and performance to determine the technical feasibility of the planned mission.