We examine the effect of adatom diffusion on the size broadening of nanostructures in laser-focused deposition by considering two major diffusion processes, namely, terrace diffusion and step-edge descending. We find that adatom diffusion sets a limit of the structure size no matter how well focused the atomic beam. A criterion is derived from rate-equation analysis to predict the limiting size of the structure, which is further confirmed by kinetic Monte Carlo simulations. The criterion suggests that, for a given growth, system, the structure feature size can be minimized by increasing deposition flux rate or by decreasing growth temperature. Applications of the criterion to existing experiments are also discussed.