The evolution of a nanosized particle in colloidal solution has been studied theoretically. The model developed is based on the size dependence of the activation energies of the growth and dissolution processes and takes into account the mass transport dynamics. Using this model, Monte Carlo simulations of the evolution of an ensemble of growing nanoparticles during Ostwald ripening have been performed, and conditions leading to either "focusing" or "defocusing" of the particle size distribution were found. The stationary particle size distribution inherent to the Ostwald ripening process in ensembles of nanoparticles less than 5 nm in radius is narrower and more symmetric than that predicted by the Lifshitz-Slyozov-Wagner theory valid for ensembles of larger (> 20 nm in radius) colloidal particles. The growth of nanoparticles in the diffusion-controlled regime results in better final size distributions as compared with those grown in the reaction-controlled regime. The dependence of the particle size distribution on a number of external and internal ensemble characteristics is studied and possible ways of controlling the particle size distribution are discussed.