Plasticity size effects offer both measurement challenges and a material engineering opportunity. We have used nano-indentation to study the relationship between size effects. Hardness varies significantly with indent size in single crystals and, in polycrystals, whenever indent sizes and structure sizes are within an order of magnitude of each other. We show, by exploiting geometric self-similarity of a Berkovich indenter and applying slip distance theory, that indent size, grain size, and pinning defects combine in a single, length-scale dependent deformation mechanism, to determine the yield strength (hardness) of a material. This provides an excellent foundation for: improved grain size determination by indentation, design rules for combining different methods of yield stress enhancement, and using indentation to probe local stress-strain properties of a material, or when mapping residual stress.