This study presents a comprehensive 3D Finite Element Analysis (FEA) to investigate the geometric scaling effects and failure mechanisms in woven carbon-epoxy adhesive joints. While Single Lap Joints (SLJs) are widely used, Single Strap Joints (SSJs) offer aerodynamic advantages but suffer from severe eccentricity-induced peel stresses. The primary objective was to numerically elucidate why increasing the adherend thickness in SSJs leads to a drastic reduction in structural efficiency. The numerical models utilized Cohesive Zone Modeling (CZM) [1] to simulate the adhesive layer and the Hashin damage criterion [2] to predict intralaminar failure within the composite adherends. The FEA results successfully captured the transition in failure modes observed experimentally. For thin (8-ply) adherends, the model predicted a cohesive-dominated failure with high fiber utilization, correlating with the "Fiber Tear" mode. In contrast, for thick (12-ply) adherends, the model revealed a premature structural failure driven by excessive peel moments. A key finding was the under-utilization of the adhesive capacity in thicker joints; the peak shear stresses at failure were significantly lower compared to the thinner counterparts (6 MPa vs. 11 MPa). This confirmed that the transverse strength limits of the composite adherend were exceeded before the adhesive could develop its full load-bearing potential. The damage evolution contours (CSDMG) accurately predicted the shift from a distributed stable fracture in SLJs to an unstable, abrupt delamination in thick SSJs. These numerical insights provide critical guidelines for designing scaled composite joints, demonstrating that geometric eccentricity, rather than adhesive strength, is the limiting factor in thick Single Strap configurations.