Sub-wavelength Fabry-Perot like resonators are studied both in reflection and transmission for the purpose of second order frequency conversion. The latter are able to hugely confine incoming electric field at resonance inducing great quantity of non linear polarization and thus resonant Sum or Difference Frequency Generation. A metamaterial model is used to homogenize the structure composed of an alternation of non linear dielectric crystal and of metal to predict its resonance wavelengths. The subsequent effective non linear susceptibility for the homogenized layer is driven by the nonlinearities of the dielectric material and by the geometrical parameters, thus leading to much higher susceptibility than existing materials. Besides, the obtained frequency spectra offer a great visibility on the various mode matching scenarios that allow to reach enhanced non linear efficiency highly depending on whether the produced wave is back- or forward propagating.