Amorphous sputtered Co-based thin films are widely used as soft magnetic materials in applications such as sensors, inductors and magnetic flux concentrators. The magnetic properties of these films can be controlled by deposition parameters like film thickness, argon pressure, deposition rate and others. In this study, we present a detailed investigation of the magnetic properties of RF-sputtered Co₉₀Nb₈Zr₂ films with thicknesses ranging from 52 nm to 1040 nm. These amorphous films exhibit an average saturation magnetisation of (1.01 ± 0.04) MA/m. As the film thickness increases, there is a significant decrease in coercivity, remanent-to-saturation magnetisation ratio M_r/M_s, and maximum permeability. The change in macroscopic magnetic properties is also reflected by the domain structure. At a thickness of 52 nm, the remanent domain state shows irregular domains, while films thicknesses above 208 nm exhibit flux-closure domain structures instead. The thickness-dependent modifications are attributed to the transition between Néel and Bloch type domain walls, which is expected to occur at approximately 84 nm.