Why should the spacings of the wires that constitute a Faraday cage be smaller than the wavelength of the radiation we wish to prevent from penenetrating inside the cage?
I think I've found it. According to the Babinet principle, the slot radiates the same way a metal sheet would radiate if it had the same dimensions as the slot. But with dimensions much smaller than a wavelength this radiation is very faint. Thus, the interior of the Faraday cage is protected.