Planktonic microorganisms are affected by various size-dependent processes both from the bottom up and from the top down. We developed a simple resource-consumer model to explore how size-dependent resource uptake and resource loss influence the growth of, and competition between, planktonic microorganisms. We considered three steps of resource uptake: diffusive transport of resource molecules, uptake by membrane transporters, and cellular enzymatic catalysis, and we investigated optimal cell size when one, two, or three of those steps limit resource uptake. Optimal cell size depends negatively on the size of resource molecules when resource uptake is limited by diffusive transport and membrane uptake. When competing for two resources of different molecular sizes, two different-sized consumers can coexist if the inputs of resources and sizes of consumers are correctly chosen. The model suggests that mixtures of various-sized resources can promote coexistence and size diversity of microorganisms even if the availability of one element, such as carbon, nitrogen, or phosphorus, limits the whole community. Model predictions include that bacteria grown on maltose or polysaccharides should be smaller compared with those grown on glucose under carbon limitation. Our results suggest that size of resource molecules can be an important factor in microbial resource competition in aquatic environments.