Microcosmic is the adjective that belongs with microcosm. You use it when a small group, place, or moment has the same qualities as a much larger one. A microcosmic picture of a country might be one family, one shop, or one village that seems to stand for the whole. It is a formal, essay-like word; you will not hear it much in everyday talk. It looks dangerously close to microscopic, which means “too small to see” or simply “tiny.” Those two share the micro- part, but they do not mean the same thing. Though actually, writers sometimes reach for microcosmic because it sounds clever; the kind question to ask is whether the small thing really does carry the big one.