Planktonic microorganisms undergo various size-dependent bottom-up and top-down processes. We developed a simple resource-consumer model to explore how size-dependent resource uptake and mortality processes affect the growth of, and competition between planktonic microorganisms. We considered three processes of resource uptake: diffusive transport of resource molecules, uptake by membrane transporters, and enzymatic catalysis within a cell, and investigated conditions for the existence of a positive optimal cell size when one, two, or three of those processes limit resource uptake. The optimal cell size depends negatively on resource molecule size when resource uptake is limited by diffusive transport and membrane uptake. When competing for two different-sized resources, two different-sized consumers can coexist if the inputs of two resources and consumer cell sizes are correctly chosen. The model suggests mixture of various-sized resources can promote coexistence and size diversity of microorganisms even though availability of one element, such as carbon, nitrogen, or phosphorus, limits the whole community. The model predictions include that bacteria grown with maltose or polysaccharides should be smaller than their size compared to those grown with glucose and that size diversity increases when incubated in mixture of various-sized organic matter. Our results suggest that size of resource molecules is an important factor in microbial resource competition in aquatic environments.