The authors address feature-size dependence in Ar/Cl₂ chemically assisted ion beam etching (CAIBE) in the context of the fabrication of photonic crystal (PhC) structures. They systematically investigate the influence of various parameters such as hole diameter (115-600 nm), etch duration (10-60 min), and ion beam energy (300-600 eV) on PhC etching in InP with Ar/Cl₂ CAIBE. For a 60 min etching at an Ar-ion energy of 400 eV, the authors report an etch depth of 5 μm for hole diameters d larger than 300 nm; the etch depth is in excess of 3 μm for d larger than 200 nm. The evolution of roughness at the bottom of the etched holes and its dependence on hole size and etching conditions is discussed. The physical mechanism of the observed feature-size dependent etching (FSDE) is then discussed and the effect of the process parameters is qualitatively understood using a model combining the effect of ion sputtering and surface chemical reactions. Finally, the effect of FSDE on the PhC optical properties is assessed by measuring the quality factor of one-dimensional Fabry-Pérot PhC cavities. The measured quality factors show a clear trend with the etch depth: the cavity Q increases as the etch depth increases