We calculate the cumulative width w of ductile shear zones accommodating plate motion in continental lithosphere, based on the assumptions that (1) the flow stress is controlled by the yield strength of intact rock at any given depth; (2) the yield strength profile through the crust can be constrained from observations in exhumed shear zones; and (3) strain localization is primarily caused by grainsize reduction leading to a switch to grainsize-sensitive creep. We use a mid-crustal stress-temperature profile measured in the Whipple Mountains, California, and calculate stress profiles at depth from published flow laws for feldspar and olivine. We conclude thatwfor a plate boundary shear zone accommodating 50 mm/yr displacement (comparable to the San Andreas Transform) may reach 180 km in the quartz-rich mid-crust, depending on water fugacity and thermal gradient. It narrows to a few meters in feldspathic lower crust and in the uppermost mantle, and then widens rapidly with depth in the lower lithosphere. We explore the effects of differences in crustal thickness and composition, thermal gradient, and water activity.