Abstract: To clarify the dimensional effects on the TL fracture behavior of European spruce, this study systematically investigated the influence patterns of three critical parameters-crack length, specimen height, and specimen thickness - through orthogonal experimental methodology. A three-factor three-level orthogonal array was employed in the experimental design, involving the preparation of 16 sets of standard specimens. Fracture processes were monitored using a double cantilever beam (DCB) testing configuration combined with digital image correlation (DIC) technique. The results revealed that the TL fracture process exhibited four distinct phases: linear elastic stage, crack initiation stage, stable crack propagation stage, and unstable failure stage. Orthogonal parameter analysis demonstrated that specimen thickness emerged as the dominant factor affecting fracture toughness, with both the fracture stress intensity factor and fracture strain energy release rate showing increasing trends followed by stabilization as thickness increased. These findings provide theoretical foundations for fracture-safe design of critical load-bearing joints in timber structures, while offering scientific guidance for optimizing cross-sectional dimensions of wooden components to balance material efficiency with fracture resistance requirements.