Abstract: In the study of novel lattice-web reinforced composite foam sandwich structures as ship impact protection materials, a recurring concern arises due to the fact that experimental specimen dimensions are often significantly smaller than those of actual engineering structures. This discrepancy casts doubt on whether laboratory-scale results can accurately reflect the mechanical response and energy dissipation characteristics of large-scale structures—namely, whether a size effect exists. To effectively bridge the scale gap between laboratory test results and real-world engineering applications, and to enhance the engineering applicability of research findings, this paper systematically conducts a size effect analysis of composite foam sandwich cylinders using numerical simulation methods based on Bazant’s size effect law within the ANSYS/LS-DYNA finite element platform. Specifically, this study first establishes refined numerical models for small-scale specimens and validates them against experimental results. On this basis, lateral compression numerical simulations are carried out for composite foam sandwich cylinders across different scales. By comparing key indicators—such as peak load, energy absorption capacity, and damage evolution patterns—among cylinders of varying sizes, the influence of the size effect on the lateral compressive performance of composite sandwich cylinders is elucidated. Further investigation based on Bazant’s size effect theory reveals that the nominal strength of all sandwich cylinders is size-dependent. The transitional size D₀ for the steel-reinforced type is only 14.6% and 22.3% of the values for the pure and ceramsite-filled types, respectively, demonstrating that the introduction of steel reinforcements leads to the most significant size effect among the three configurations. These findings can provide theoretical support for the optimized design and scale extrapolation of bridge anti-collision structures.