Clean planar Ir(210) and clean nano-faceted Ir(210) with tunable sizes have been prepared from the same crystal in situ, which allows us to investigate structure sensitivity and size effects in surface chemistry on unsupported iridium without switching from sample to sample. A series of chemical reactions have been carried out on clean planar and clean faceted Ir(210) using temperature programmed desorption (TPD) measurements. We have found clear evidence for structure sensitivity in decomposition of acetylene, nitric oxide, ammonia and hydrogen, and for oxidation of CO on planar Ir(210) versus faceted Ir(210). Our measurements of CO oxidation over planar and faceted Ir(210) are supported by our DFT calculations of the energetics of adsorption and diffusion of oxygen; the results suggest that surface diffusion may play a role in controlling CO oxidation over Ir. In addition, ammonia decomposition on faceted Ir(210) exhibits size effects on the nanometer scale, which is the first observation of size effects in surface chemistry on unsupported monometallic catalyst with well defined structure and controlled size.