The truss-braced wing (TBW) is a promising configuration for enabling high-aspect-ratio wings while alleviating wing-root bending loads through alternative axial load paths. Existing literature shows that structural modeling of truss-braced wings is largely dominated by two extremes: (i) numerical beam-based multidisciplinary design optimization frameworks, which are computationally efficient but analytically opaque, and (ii) high-fidelity finite-element models, which are accurate but computationally intensive. What remains largely absent is a lightweight, parametrized analytical structural model that captures bending–torsion coupling and enables a transparent structural assessment of different truss configurations during the conceptual and preliminary design phases. This work addresses this gap by developing a fully parametrized structural framework for a modular truss-braced wing based on geometrically exact beam theory. The wing–truss system is modeled using a geometrically exact Simo–Reissner bending–torsion beam formulation, capturing coupled axial force, shear force, bending moment, and torsional moment while explicitly representing the truss as a discrete load-carrying member. Realistic spanwise aerodynamic and mass distributions are applied under representative maneuvering load cases. A parametric study is performed to investigate the influence of truss attachment location in both the spanwise and chordwise directions, quantifying how truss placement governs internal load redistribution along the wing. Subsequently, wingbox structural sizing is carried out using both the Simo–Reissner beam formulation and detailed finite-element analyses in MSC Nastran, yielding consistent thickness and mass trends. In addition, the truss is structurally sized under axial load and buckling constraints, and its aerodynamic performance is evaluated through the associated drag contribution. The proposed framework provides a physically transparent and computationally efficient tool for conceptual and preliminary TBW design, supporting aero-structural trade-off studies prior to higher-fidelity numerical analyses.