The invention concerns a size-determining system and method of size grading type, used in particular to determine sizes, and a method to produce said system for a product to be worn on parts of the body, notably in the area of items for sub-aquatic vision. The method to produce the system comprises selecting principal morphological parameters from among parameters corresponding to the distance between two morphological markers characteristic of the body part. It also comprises selecting product parameters corresponding to the distance between two geometrically characteristic points of the product, determining an arithmetical relationship between each product parameter and the principal morphological parameters, and determining sizes defined by particular variation intervals of the principal morphological parameters or product parameters. The system comprises means to store values of the morphological parameters and product parameters, a module to calculate the value of product parameters in relation to the value of the principal morphological parameters, and means to store sizes defined by particular variation intervals of the principal morphological parameters or product parameters. The method comprises calculation of the product parameter values, in relation to the values of the principal morphological parameters which are determined by selecting a size from a group of sizes defined by variation intervals of the principal morphological parameters, and by assigning median values of the intervals to the corresponding principal morphological parameters.