<p>Thin coatings have emerged as a critical component of advanced nuclear fuels. Located in the path of heat dissipation, a thorough investigation of coating thermal conductivity is imperative. The spatial-domain thermoreflectance (SDTR) technique is ideally suited for characterizing their thermal conductivity due to its high spatial resolution. However, applying SDTR to cross-sectional samples is complicated by their asymmetric geometry and layered structure, which preclude analytical heat transfer solutions and introduce significant uncertainty in the data analysis. Here, we employ finite element modeling, validated by experiments, to quantify the size effects that govern SDTR measurements on cross-sectional coating samples. We determine the minimal coating dimensions required for accurate, artifact-free measurement and reveal how these dimensions are influenced by the thermal property mismatch between the coating, substrate, and transducer film. We further quantify the measurement error induced by off-center laser positioning. Through systematic investigation of progressively complex scenarios, analytical expressions for rapid determination of minimum dimensions free from boundary artifacts are derived. Our framework establishes practical guidelines for accurate thermal characterization of coatings by SDTR for energy materials research.</p>