Solutions of nanoparticles can be used as a general platform to design glues for polymer-based gels. However, optimization of the system for a specific gel type requires understanding how different experimentally tunable parameters concur to determine the gel–gel interfacial adhesion. Here, we use molecular dynamics simulations to investigate the effect of nanoparticle size and gel–nanoparticle interaction for a coarse-grained model representative of hydrogels, or in other words a cross-linked polymer network in a swollen state, of varying stiffness. We show that, regardless of the stiffness, competing effects lead to an optimal nanoparticle size for interface reinforcement that is always around the polymer mesh size. We also show that, moving away from this optimal size, the glue performance can quickly degrade to the point that nanoparticles can even weaken interface adhesion, suggesting a limit to the amount of polydispersity in the particle size that can be accepted to obtain a functional glue. Overall, these simulations provide a further step toward the rational design of nanoparticles solutions with optimal gluing properties.