Reproducible control of the magnetic vortex state in nanomagnets is of critical importance. We report onchirality control by manipulating the size and/or thickness of asymmetric Co dots. Below a critical diameterand/or thickness, chirality control is achieved by the nucleation of a single vortex. Interestingly, above thesecritical dimensions, chirality control is realized by the nucleation and subsequent coalescence of two vortices,resulting in a single vortex with the opposite chirality as found in smaller dots. Micromagnetic simulations andmagnetic force microscopy highlight the role of edge-bound half vortices in facilitating the coalescence process.