The geometric sizes of the players important in soil organic matter decomposition (microbes, exoenzymes, polymers, monomers) vary over orders of magnitude, and this feature has not been considered in biogeochemical kinetics. We describe here a theory based on diffusion and law of mass action to analyze how sizes of organic polymers, exoenzymes, organic monomers, and microbial cells interact to affect emergent chemical kinetics and their temperature and moisture sensitivities. We predict that because polymers are often much larger than exoenzymes, their relationship is more of the reverse Michaelis-Menten type, while because microbial cells are much larger than monomers, their relationship follows more closely the forward Michaelis-Menten kinetics. However, both types of interactions can be well approximated with the Equilibrium Chemistry Approximation kinetics. Using the temperature and moisture dependence of diffusivity, forward reaction rates, and protein stability, we predict that (1) exoenzymes are probably thermally stable over most terrestrial temperature ranges, such that their depolymerization rate temperature dependence is monotonic and of the Arrhenius type, (2) temperature dependence of the exoenzyme substrate affinity parameter is almost of the Arrhenius type with deviations caused by diffusion, and (3) the temperature dependence of monomer uptake is bell-shaped. We show that the magnitudes and functional responses of our predictions are generally consistent with reported measurements. Further, we show that that temperature and moisture sensitivities of the modeled kinetic parameters cannot be accurately described with products of simple functions, as is commonly done in models. We expect our results can bring new insights to how soil carbon dynamics will respond to expected 21^stcentury changes in temperature and moisture.