<p>This paper presents a sizing and design methodology for the conceptual phase of blended wing body (BWB) aircraft aerodynamics. The proposed design framework provides a sized and trimmed lifting body with predefined longitudinal stability. Starting from a specified sizing point and cabin geometry, a four-segment lifting body planform is constructed. The fuselage segment is defined by encasing the cabin, while the remaining lifting surface area is allocated to the wing and sized according to the required wing loading. The methodology incorporates distinct longitudinal stabilization and trimming processes. The aerodynamic center is primarily determined by the lifting body planform, allowing an independent definition of the aerodynamic behavior based on airfoil selection. To achieve desired stability margins, the wing is relocated in relation to the fuselage in an iterative process. To minimize vortex lattice method executions, a Gaussian Process Regression serves as a surrogate model in determining the aerodynamic center location. Tailored fuselage airfoils provide both positive pitching moments and lift through variations in camberline and thickness distribution. The iterative trimming process ensures that the aerodynamic behavior meets the design requirements while fitting the cabin within the fuselage airfoils at all times. Ultimately, this methodology yields a sized and trimmed BWB lifting body with well-defined longitudinal stability characteristics, enabling effective exploration of the BWB design space. The paper is limited in scope as it does include simplified mass models, no structural boundary conditions or consideration of flight dynamic lateral motions.</p>