- Mechanism-based discrete dislocation plasticity is used to investigate the effect of size on micro-scale crystal plasticity under nominally uniform compression. Constitutive rules are used which include a physical representation of dislocation sources and obstacles in the undeformed samples. The simulations predict a strong size dependence of the material flow stress and reproduce the main features of recent experiments. Possible operating mechanisms are identified which highlight the interplay between sample dimensions, internal stresses and the length and time scales inherent to dislocation generation and motion. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.