Porphyrin and phthalocyanine transition metal complexes adsorbed on activated carbon fiber nanoporous supports are shown to be effective catalysts for CO2 electroreduction in the form of gas diffusion electrodes, yielding carbon monoxide with current efficiencies up to ca. 70%. A comparison of the efficiency of activated carbon supports with a wide pore size distribution, and activated carbon fibers, with slit-shaped pores of ca. 2 nm width, demonstrates that an increase in the catalyst dimensions results in a decrease in the benefit of the so-called nanospace effect. For bulky molecules, e. g., tetra-tert-butyl-substituted phthalocyanines, activated carbon supports are more effective, with a current efficiency as high as 85% for CO production being achieved.