Herein, we investigated the effect of Pd size on a formic acid dehydrogenation (FAD) reaction. A series of size controlled Pd/C catalysts was simply prepared by a treatment using sodium borohydride (NaBH4), which provides in-situ hydrogen in the aqueous phase. Pd particles in the catalyst were gradually agglomerated as the number of NaBH4 treatments increased, resulting in the synthesis of the size-controlled Pd/C catalysts with the Pd size from 2.5 nm to 4.8 nm. The catalytic activities of the size-controlled Pd/C catalysts continuously declined with increasing the Pd size, which was attributed to the decrease in the number of active sites of the catalyst. However, the decreasing trend of turnover frequency (TOF(h(-1))) values with the Pd size at low temperature demonstrated that the intrinsic activity of the small Pd is higher than that of the large Pd. More importantly, such decreasing trends of TOF values as a function of the Pd size gradually disappeared when raising the reaction temperature. It implies that there is a thermodynamic effect of the Pd size on the FAD reaction, which was evidenced by a continuous increase in the activation energy from 44.9 kJ/mol to 63.9 kJ/mol with the Pd size. We proposed that the H-2 desorption step is the rate-determining step for FAD reaction. Such proposal can explain the size-dependent catalytic activity and the increase in the activation energy of FAD with the Pd size enlargement, as the temperature programmed hydrogen decomposition (TPHD) analysis demonstrated that the large Pd particle requires a high temperature/energy to desorb hydrogen from Pd. Consequently, it is concluded that the size-dependent catalytic activity of FAD is attributed not only to the number of active sites but also to the thermodynamic factor relating to the rate-determining step.