A crystal plasticity elasto-viscoplastic FFT (Fast Fourier Transform) formulation with a mesoscale fied dislocation mechanics model (“MFDM”) is presented. The present MFDM-EVPFFT approach accounts for plastic flow, hardening and densities of geometrically necessary dislocations (“GND”), in addition to statistically stored dislocations (“SSD”). Here, the model incorporates a defect energy density that depends on GND densities. This allows to thermodynamically derive internal length dependent intra-crystalline backstress and Peach-Koehler force acting on GND densities. The model considers GND density evolution through a filtered numerical spectral approach, which is coupled with stress equilibrium through the elasto-viscoplastic FFT algorithm. The discrete Fourier transform (DFT) method together with finite difference schemes is applied to solve both the lattice incompatibility problem and the Lippmann-Schwinger’s equation. Numerical results are first reported for two-phase channel-type composites with plastic single crystal channels and elastic precipitates for shear loadings. Channel size effects are simulated and analyzed on the overall and local hardening behaviors during monotonous loadings. In addition, the evolutions of GND densities and the role of their associated backstress on size effects are examined during reversible loadings.