Thiolated dendritic mesoporous silica nanoparticles (T-DMSNs) with ultrahigh density of thiol groups (284.6 ± 9 μmol g-1) are synthesized and used to load gold nanoparticles with tunable sizes (1.2-2.7 nm) and high content (34.0 wt %). It is demonstrated that the size of gold nanoparticles has a significant impact on their peroxidase-mimicking activity. At an optimized size of 1.9 nm, T-DMSNs@Au exhibits the highest activity. Our contribution provides new insights into the rational design of nanozymes for future applications.