A method for determining a size grid, the method comprising following steps: a) determination of N morphological parameters, N being ≥2; b) acquisition of a set of values of the parameters for a sample of a targeted population of individuals, each individual being associated with an individual point supplying, for each parameter, a value of the parameter; c) independently of steps a) and b), determination of rate of coverage of sample to be covered by the grid; d) determination of a set of coverage zones, each coverage zone being a set of individual points relating to a set of individuals for which a same orthesis according to the model is adapted and thus corresponding to a size adapted to set of individuals; the number of coverage zones being determined such that percentage of individual points included in at least one coverage zone is ≥ the desired coverage rate.