[en] Using the dislocation–kinetic approach based on equations describing the evolution of the average dislocation density in a material with an increase in shear strain, size effects have been theoretically studied that are associated with the effect on the strain hardening of the sample of the average grain size d_S and thickness D and are caused by the dependence of the kinetic equation terms on d_s and D. Established in a common way using the Taylor relation and experimentally confirmed the relationship between the flow stress σ and the strain degree ε was used to analyze the strain hardening of flat samples of two–dimensional high–purity Al polycrystals with different grain sizes in the range ~ 50μm < d_s < ~ 1mm and various thicknesses in the range of ~ 50μm < D_s < ~ 1mm under uniaxial tension with a constant strain rate at moderate temperatures. It was shown that the dependence of the offset yield strength σ_0.2 on the grain size d_s almost vanishes at d_s > 300μm. It was found that the values of σ_0.2 and the strain-hardening coefficient θ = d_σ / d_ε decrease with increasing thickness D of the sample. These decreasing dependences are replaced by increasing ones with increasing degree of deformation. Stress σ and coefficient θ increase with increasing ratio D / d_s at all stages of deformation. (author)