This study investigates the impact of nanoparticle diameter on film boiling in Al ₂O ₃ water-based nanofluids along a vertical cylinder, focusing on nanoparticle sizes of 5 nm, 10 nm, 30 nm, and 50 nm. Using a Continuous-Species-Transfer method within a Computational Multi-Fluid Dynamics framework, the behavior of nanoparticles in both liquid and vapor states is simulated, considering Brownian motion and thermophoretic effects. A 2D axisymmetric analysis reveals how nanoparticle sizes influence temperature gradients, nanoparticle concentration patterns, thermophysical properties, and the Nusselt number to assess heat transfer efficiency. Findings indicate that during film boiling, smaller nanoparticles exhibit higher concentrations on the heated wall over time. Additionally, nanofluids with smaller particles demonstrate higher thermal conductivity and viscosity near the heated wall, along with lower surface tension. Interestingly, nanofluids with 30 nm and 50 nm nanoparticles perform closely, indicating a size threshold beyond which further reductions do not significantly enhance boiling heat transfer. In contrast, nanofluids containing 5 nm and 10 nm nanoparticles exhibit markedly superior thermal removal efficiency, underscoring the critical role of nanoparticle size in optimizing film boiling performance.