Nanoparticles made of transition metals have attracted increasing attention as catalysts for sustainable hydrogen economy and environmental remediation. Compared to conventional techniques, the use of nanocatalysts can shorten reaction time, target recalcitrant compounds, and selectively transform wastes into valuable products under mild conditions. An important challenge is, however, to reduce the high cost associated with the initial investment and subsequent replenishment of catalysts. A variety of catalysts have been investigated to improve the reactivity of the reactions; however, little attention has been given to the possible influence of size-effect on catalyst design. Understanding how size controls the active sites and thus determine the reactivity is the key to reduce the cost of these catalysts. Here, cobalt (Co), ruthenium (Ru) and palladium (Pd) nanoparticles with controllable sizes are synthesized for environmental heterogeneous catalysis. It is shown that Co nanoparticles can be affixed on spinel nanodisks through heteroepitaxy with thermal phase transformation. However, in-situ reduction of adsorbed cations on support and protecting nanoparticles with polymers are more readily to be used for noble metals like Ru and Pd. Using size controlled face-centered cubic (FCC) packed Ru, which have tunable activation energies from 18 to 87 kJ mol^-1, compensation effect and the resulting isokinetic temperature of T_i = 17.5(±1.6) °C was observed in ammonia borane hydrolysis to generate hydrogen. The surface area normalized rate constant, k _S, can be maximized below T_i by reducing the size of Ru-FCC nanoparticles, which increases the fraction of edge/corner atoms and lowers the activation energy. To generate hydrogen above T_i, k_S is maximized by using enlarged Ru nanoparticles. We also connect hydrogen solubility in Pd nanoparticles to their size sensitivity in hydrogenation of p-nitrophenol. The unvaried activation energy for Pd nanoparticles with different sizes suggests the unchanged active sites, which is proposed as the dissolved hydrogen activated Pd dimer. A quantitative structure-activity relationship is established to reconcile paradoxical observations of sizedependent reaction rates and a size-independent activation energy. In summary, size-effect is crucial for the design of environmental catalysts. More importantly, it helps elucidate both the active sites of the catalysts and the mechanisms of reactions.