We describe a three-dimensional, dislocation dynamics (DD) model of dislocation plasticity in BCC micro-pillars and use it to study size effects and the effects of initial dislocation density and strain rate on strength. The model is based on the molecular dynamics (MD) simulations of Weinberger and Cai who discovered a surface-controlled cross-slip process leading to dislocation multiplication without the presence of artificial pinning points. We find a smaller stronger size effect that can be understood in terms of the dependence of the mobile dislocation density on pillar size, through the balance between the multiplication rate and depletion rate (from the surface). In this respect, neither the single-arm source model nor the starvation/nucleation model accounts for the size effect. Instead, it is controlled by the multiplication/loss competition. Although DD simulations still require higher strain rates than those found in experiments, the DD simulations incorporating the surface multiplication mechanism predict the dependences of flow stress on pillar size, initial dislocation density and strain rate that are largely consistent with experiments. An analytical model is constructed to rationalize the behavior of the DD model at such high strain rates.