Flows in rotating drums play a crucial role in various industrial processes involving granular materials, such as mixing, crushing, granulation, drying, and chemical reactions. The flow pattern within the drum is determined by the Froude number determined from the angular velocity, the drum diameter, and the gravitational acceleration. When the Froude number is relatively small, a steady flow regime emerges, characterized by two distinct regions: (i) a surface flow layer, where granular materials exhibit fluid-like behavior, and (ii) a static region, where the materials rotate rigidly, following the motion of the drum wall. Numerous experimental studies have explored the dependence of the surface flow layer thickness in drums half-filled with granular materials. However, the scaling laws proposed in these studies are inconsistent, and a theoretical explanation remains unestablished. We numerically investigate the steady flow of a two-dimensional granular material in a rotating drum using both the discrete element method (DEM) and computational fluid dynamics (CFD) based on a constitutive equation. The CFD simulations successfully reproduce the coexistence of the surface flow layer and the static region, as observed in the DEM simulations. Our results indicate that the surface flow layer thickness increases with the drum diameter and exhibits a weak dependence on the angular velocity. Furthermore, we analytically derive scaling laws for the velocity profile and the surface flow layer thickness based on the continuum model used in the CFD simulations. These scaling laws are validated numerically through DEM simulations. Moreover, they are consistent with the experimental results in a previous experiment, where the cylinder is sufficiently long in the axial direction, allowing the flow to be considered two-dimensional.