In this study, the diameter-dependent behavior of smooth horizontal air-water stratified flow was investigated using three-dimensional transient computational fluid dynamics simulations. The study addresses the limited understanding of how pipe diameter modifies phase distribution and hydraulic resistance under identical superficial flow conditions. The flow was modeled using the Volume of Fluid method coupled with the SST k-ω turbulence model for horizontal circular pipes with diameters of 25.4, 38.1, 60.0, and 101.6 mm at j_G = 0.41 m/s and j_L = 0.04 m/s. Mesh and time-step independence were assessed, and the 60 mm case was validated against published experimental data, with deviations of 3.66% in void fraction and 1.38% in frictional pressure gradient. The results demonstrate that the increase in diameter systematically restructures the stratified flow: normalized liquid height decreases from 0.485 to 0.316, while void fraction increases from 0.599 to 0.775. Furthermore, the peak axial velocity decreases from approximately 1.26 to 0.71 m/s, accompanied by broader gas-phase velocity distributions and reduced turbulent kinetic energy. Most significantly, the frictional pressure gradient decreases by approximately 96%, from 2.515 to 0.101 Pa/m. This disproportionate hydraulic response relative to the void-fraction change indicates that diameter effects arise from coupled changes in phase segregation, interfacial momentum transfer, and flow-field organization rather than geometric enlargement alone. The findings provide a physically based numerical framework for evaluating diameter-dependent stratified-flow behavior relevant to nuclear thermal-hydraulic systems and other horizontal two-phase transport applications.