Despite decades of research, the nature of size effects and phonon confinement in thermal boundary resistance (Kapitza resistance) remains poorly understood. While size-dependent thermal transport is well documented in homogeneous crystalline systems, the assumption that identical size-dependent phenomena occur in these fundamentally different systems lacks strong support. In fact, what evidence does exist often turns out to be either simulation artifacts or misleading interpretations arising from inadequate model assumptions.  First-principles simulations of interface systems only a few nanometers long yield Kapitza resistance values that match those obtained from semi-infinite (Greens Function) simulations and agree well with experimental data. This suggests that interfacial size effects, while likely real, manifest in fundamentally different ways from their homogeneous, crystalline counterparts. To our knowledge, these interfacial-specific size effects have not been systematically investigated. This work presents a focused study of size-dependent interfacial phonon behavior, with implications for understanding heat transport in nanotechnology and in next-generation semiconductor devices.