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Spatial scales in crystal plasticity are understood to influence flow stresses and work-hardening rates. A direct assessment of the crystal-size dependence of the critical resolved shear stress has been made for single-slip oriented crystals of pure Ni having sample diameters that range from 40 to 1.0μm. The sample dimensions directly limit the length scales available for plasticity, but without significant external or kinematical constraint. The results show strength increases of up to 15 times over pure Ni and size-affected hardening rates. Stresses are lower, but strengthening persists to larger sizes than for the prior work on face-centered cubic metal whiskers. The results emphasize that at the micron-size scale and below both external geometry and internal structure affect the micromechanisms of deformation and strength.