The structural relaxation caused by the plastic deformation of polymer glasses is not yet fully understood. In particular, the size scale of the localized plastic relaxation events is currently unknown. In this work, the effect of molecular entanglement density on the shear activation volume of glassy polymers has been studied. The shear activation volumes of miscible polystyrene-poly(2,6-dimethyl-1,4-phenylene oxide) (PS-PPO) blends at different PS/PPO ratios have been determined experimentally by both plane-strain and uniaxial compression at constant strain rates. We find that the same correlation between the shear activation volume V-eq* and the entanglement density rho(e) holds for the blend as well as for various pure glassy polymers: V-eq* = C(rho(e)/nm(-3)), + V-0, with C = 8.2 +/- 0.4 nm(3), alpha = 0.6 +/- 0.03, and V-0 < 0.1 nm(3) eq. The shear activation volume is closely related to the size of the plastic shear zones; therefore, this correlation suggests that the cooperativity of the elementary processes of plastic deformation in glassy polymers scales with rho(e).