Optical measuring systems for measuring geometrical parameters of nano-objects and methods of measuring a critical size (CS) are provided. The optical method of measuring the CS includes selecting parameters of an optic scheme and an illumination condition; recording a set of nanostructure images corresponding to various wavelengths with various defocusing levels of scattered radiation; calculating a plurality of sets of images of a nanostructure with various defocusing levels, corresponding to various wavelengths of the scattered radiation with CS values within a known range; and comparing a set of measured images of the nanostructure with the sets of the calculated images and determining a best approximate value of the CS values.