Island densities and island sizes can be varied or even tailored by choice of external parameters in molecular beam epitaxy, i.e.: by the choice of substrate temperature. deposition rate, and coverage. We present a comprehensive study of the nucleation kinetics of Cu on Ni(100) using variable temperature scanning tunneling microscopy. The analysis of the saturation island density as a function of substrate temperature and deposition rate reveals that the smallest stable island abruptly changes from a dimer to a tetramer. The sizes of the critical nuclei are determined from the rate dependence of the saturation island density using mean-field nucleation theory consistent with results from the island size distribution using scaling theory. From the Arrhenius-plot, the microscopic quantities (migration barrier and dimer bond energy) have been deduced.