Recent micro-pillar experiments have shown strong size effects at small pillar diameters. This ‘Smaller is stronger’ phenomenon is widely believed to involve dislocation motion, which can be studied using dislocation dynamics (DD) simulations. In the present study, we make a 3-D DD model to study the collective dislocation behavior in BCC micro-pillars under compression. We consider a surface-controlled cross-slip process, involving image forces and non-planar core structures, that leads to multiplication without the presence of artificial pinning points. We follow both the evolution of the dislocation structure and the corresponding stress-strain relation. Our simulation results show the evident size effect and the effect of initial dislocation density and strain rate, which appear to be good agreement with recent experimental results.