A mesoscale computational plasticity model, which concurrently couples dislocation dynamics and the finite element method, has been developed to investigate fundamental deformation mechanisms and the corresponding mechanical response of single-crystalline aluminum porous micropillars. In this study, we explore the deformation mechanisms for incipient plasticity and the size-dependent strengthening properties in nanoporous metals under the complex loading conditions from the porous geometry. At a small scale where plastic deformation is governed by the operation of dislocation sources, the presence of nanovoids could even increase the flow stress by shortening the source length, which could also effectively hinder easy glide motion. Interestingly, nanoporous micropillars could achieve even higher strength than the bulk strength, as observed in recent experiments. In this regard, the flow stress shows clear dependence on not only macroscopic features of relative density, but also microscopic features such as the size and shape of the nanovoids. Our coupled model could provide unique opportunities for predicting macroscopic mechanical properties through the fundamental understanding of microscopic deformation mechanisms, showing good agreement with recent experimental results of nanoarchitecture materials.