This research investigates the magnetoresistance (MR) behavior of Co/Cu magnetic superlattices with a fixed cobalt (Co) layer thickness of 20 Å, focusing on how variations in the copper (Cu) layer's thickness affect the MR response across a broad temperature range (4.2–300 K). This study specifically explores the influence of the Cu layer's thickness, interfacial structure, and surface morphology on spin-dependent electron scattering, which is the dominant mechanism governing the MR ratio in such multilayered systems. A detailed theoretical framework is employed, incorporating spin-dependent Boltzmann transport equations and realistic interface models, to capture how modifications in the electron reflection, transmission, and spin filtering at the Co/Cu interfaces influence MR. Numerical simulations reveal a pronounced decrease in the MR as the Cu's thickness increases from 5 Å to 150 Å, particularly at cryogenic temperatures, where ballistic and quantum transport effects are more pronounced. This trend is attributed to reduced spin asymmetry and enhanced diffuse scattering in thicker Cu layers. The theoretical results show excellent agreement with experimental measurements conducted on electrodeposited multilayers, demonstrating the validity of the model. These findings highlight the critical importance of controlling both the Cu spacer's thickness and the interface integrity to optimizing the MR performance. These insights are essential for the design of advanced spintronic devices, where precise engineering at the atomic scale is required to achieve high spin polarization and efficient electron transport.