From the recent micro-pillar experiments, it is now known that the flow stress of metallic micro-pillars increases with decreasing sample size even in the absence of significant hardening by geometrically necessary dislocations or strain gradients. To understand size dependent plasticity, several models have been proposed, but the role of the dislocation sources in submicron sample is still under debate. In the present study, we make a three-dimensional, dislocation dynamics model to study collective dislocation behavior under compression in FCC micro-pillars. Following the molecular dynamics calculation on the dislocation nucleation rate, we consider the surface nucleation as a function of stress. Additionally, truncated dislocation sources can be observed to form through the collective dislocation interaction without any artificial pinning points. Following dislocation structure and stress-strain relation, we explore the governing mechanism for sample size effect for different sample size between the operation of truncated dislocation sources and dislocation nucleation at the surface. In addition, we study the size effects and the strain rate sensitivity on strength.