Progress in the experimental nano-sciences and laser cooling has led to the manipulation of material objects with sizes and temperatures such that quantum effects start to appear. This range of sizes coincides with that of the smallest bacteria as well as of the largest viruses, and is often considered to be the boundary between living and nonliving entities. Experiments are rapidly progressing towards putting micro-organisms into quantum superposition states in a controlled manner, resulting in nontrivial quantum effects that we normally do not attribute to living systems. In order to set limits for the occurrence of such effects, I model micro-organisms quantum mechanically and investigate how relevant physical properties are modified from their classical bulk values in the presence of classical self-gravity. The results show that a transition between quantum-like and classical behavior exists right at sizes corresponding to the broad boundary between the smallest bacteria and the largest giant viruses.