While the selection of appropriate sizes for porous solid catalysts is a standard task for designing fixed-bed reactors and is demonstrated for a vast variety of examples (e.g., CO2 methanation , CO methanation , methane steam reforming and Fischer-Tropsch synthesis ), there are only few studies that systematically consider the influence of diffusion limitations on the methanation of CO/CO2 mixtures. In our previous work, we investigated the influence of the diffusion model on the dynamic behavior of methanation on the basis of simulation experiments. However, to our knowledge, there is no simulation study available in scientific literature so far focusing on the optimal pellet size, which is complemented by physical experiments. Few works exist that either exclude diffusion or only consider the methanation of pure CO or CO2 . For instance, El Sibai et al. have calculated an optimal design for a CO2 methanation plant. The reaction kinetics by Koschany et al. were used and it was assumed as a design criterion that intra-pellet transport is not limiting. The authors calculated that at the pre-defined lower limit for the catalyst pellet size of 1 mm the transport of mass and heat is optimal without unacceptably increasing the pressure drop. Zimmermann et al. extended the aforementioned work by considering internal diffusion effects and found that the optimal pellet size for methanation of pure CO2 is 2.5 mm, which again represents the lower limit. The reactant mixture was assumed to be free of CO in both studies, which allow no conclusions on the interaction between CO and CO2 based on the reported results. For methanation of CO/CO2 mixtures reactant selectivity consideration may come into focus of reaction design, which is usually steered by the size of the catalytically active crystallite or the support material . The size of the catalyst pellet is not yet used for that purpose to the best of our knowledge.