Internal arc faults in oil-immersed transformers release energy into the surrounding oil within a very short time, generating high-intensity shock waves that threaten tank integrity. Current transformer explosion-mitigation studies predominantly rely on point-source pressure models, with limited attention to the transient, strongly nonlinear pressure propagation induced by long arc discharges and to appropriate equivalent modeling strategies. In this work, an explosion-energy equivalence approach was applied to investigate the spatial-scale effects of long-arc-induced pressure waves in oil using OpenFOAM. The evolution of shock waves from the near field to the mid- and far-field was quantitatively analyzed, with particular emphasis on the effects of source spatial discreteness and pressure-generation timing on propagation characteristics. Results show that shock waves generated by an equivalent 1.25 MJ, 120 mm arc evolves rapidly from a near-field cylindrical wavefront toward an approximately spherical geometry. Unlike a point source, however, the resulting spherical wavefront exhibits pronounced directionality, with amplitude variation up to 2.86 times. The influence of pressure-generation timing on wavefront morphology is significant during wavefront formation and early propagation but becomes negligible once the propagation distance reaches approximately twice the characteristic geometric length of the source. Under consistent total energy, volume, and energy release scale, moderate spatial discretization of the pressure source has no significant effect on mid- and far-field shock-wave behavior. This study clarifies the directional pressure characteristics masked by the apparent spherical evolution of long-arc-induced wavefronts, demonstrates the limited influence of spatial discretization and timing differences and provide an improved modeling basis for transient pressure analysis of transformer oil tanks.