The effects of specimen size, Hall–Petch (H-P) grain or subgrain size, particle size plus spacing, and crack size on the yield strength, plastic deformation, and fracturing properties of crystalline materials are described on a dislocation mechanics basis. The size effects are assessed at relevant macro- and/or micro-and/or nano-scale dimensions; in the latter case, at the upper-limiting strength levels. The description is applied mostly to face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP) metals but also involves grain size/particle size–dependent (composite) steel material behaviors. Competition is described for the role of dislocation pile-ups versus hole-joining mechanisms for ductile failure. Grain size–dependent microhardness and strain rate sensitivity measurements are presented for nano-grain size strengthening and grain size weakening, respectively. An intrinsic size effect is demonstrated for silicon crystal nano-indentation hardness testing, which, on microscale loading, leads to evaluation of crack size dependence and, for polycrystalline alumina, to associated H-P behavior for the fracture mechanics stress intensity.