We simulate the spin torque-induced reversal of the magnetization in thin disks with perpendicular anisotropy at zero temperature. Disks typically smaller than 20 nm in diameter exhibit coherent reversal. A domain wall is involved in larger disks. We derive the critical diameter of this transition. Using a proper definition of the critical voltage, a macrospin model can account perfectly for the reversal dynamics when the reversal is coherent. The same critical voltage appears to match with the micromagnetics switching voltage regardless of the switching path.Magnetization reversal in small particles is a long standing problem 1,2 that was recently put in a new context by the emergence of spin transfer torque (STT) magnetic random access memories 3 (MRAM). This technology is based on the STT-induced manipulation of the magnetization of nano-sized ultrathin disks with perpendicular magnetic anisotropy (PMA). In addition to its fundamental interest, the switching dynamics is of paramount application importance as it determines many of the performance metrics of this technology. Unfortunately experimental investigations are scarce 4-10 probably because of the technical difficulties associated with the small dimensions as well as the large frequencies involved in the switching process. As a result the switching paths are often conjectured from reasonnable but approximate models 11,12 , if not from overly simplified models like the macrospin picture 13-15 whose range of validity is still to establish.Here we unravel the size dependence of the switching dynamics by taking advantage of the accurary of micromagnetics. We first clarify how to implement STT in a way that is adequate for magnetic tunnel junctions (MTJ). We then describe the main switching regimes: coherent for small disks versus domain wall (DW) based at larger diameters. We discuss the critical diameter that separates these two regimes. We then parametrize the macrospin model to account exactly for the coherent regime. Finally we describe the size dependence of the switching voltage and provide a model valid whatever the switching mode. Our results clarify the predictive capability of the corpus of theories based on the macrospin model and therefore it has strong implications for magnetic random access memories.We are interested by the response of PMA disks to the STT associated with voltage steps applied through a tunnel junction. A first difficulty arises from the fact that the STT is most often expressed in units of current densities 18,19 while the applied voltage is a more correct metric in a tunnel junction context. Indeed the insulating nature of the tunnel oxide renders the voltage laterally a) Electronic mail: paul.bouquin@u-psud.fr uniform across the disk, while the current density is not. To implement STT within micromagnetics, we start from the Landau-Lifshitz-Gilbert-Slonczewski equation:where m is the normalised magnetization, γ the gyromagnetic ratio, H eff the effective field, and α the damping constant. We consider a spin polarization alon...