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To study macroscopic systems with coarse grained simulations one typically simulates a micro- scopic part of this macroscopic system. By reducing the size of the simulated system one introduces finite size effects. In this work we study the finite-size effects in the reaction ensemble, which is used to simulate reactive system. We calculate the finite-size effects in a non-interacting systems by explicitly calculating the partition function. This approach provides high precision data at low computational costs. For a grand canonical insertion/deletion of a pair of particles our results reproduces previously published results, validating our approach. Further, we show that a sim- ple isomerization reaction is not affected by finite size effects. For a decomposition reaction we show that previous estimates were overestimating the finite-size effects, and one can simulate much smaller systems while avoiding the finite-size effects. For previously studied acid-base equilibria the finite-size effects are only relevant at extreme conditions. The tool we provide allows to a priori estimate the finite-size effects and find the limits of the applicability of the reaction ensemble.