1. A sizing system of the grading type, which can be used in particular to determine one or more sizes, for a product intended to be worn on at least one part of the body, the product presenting pre-selected geometrically characteristic points, the part of the body presenting pre-selected characteristic morphological markers comprising: a means for storage of respective values of one or more characteristic morphological parameters (PM i,j ) corresponding to a distance between two characteristic morphological markers (M i , M j ) of said part of the body, at least one of said characteristic morphological parameters (PM i,j ) being a principal morphological parameter (PMP i,j ); a means for storage of the respective values of one or more geometrically characteristic product parameters (PP i,j ) corresponding to a distance between two geometrically characteristic points (NP i , NP j ) of said product; a first module for calculating a value of each characteristic product parameter (PP i,j ) according to the value of at least one of said principal morphological parameters (PMP i,j ); a second module for calculating the value of coordinates of each geometrically characteristic point (NP i ) of said product according to at least one of said geometrically characteristic product parameters (PP i,j ), said coordinates being expressed in a reference system centered on a particular point (NP ref ) from among said geometrically characteristic points (NP i ) of said product, a means for storing at least one sized determined by respective particular variation intervals of said principal morphological parameter or parameters (PMP i,j ), or by respective particular variation intervals of said geometrically characteristic product parameter or parameters (PP i,j ), and wherein said first calculation module is designed to perform the following calculation for each geometrically characteristic product parameter (PP i,j ): 
 PP i,j =c 0 i,j +c 1 i,j ×PMP i1,j1 +c 2 i,j ×PMP i2,j2 +. .. +c n i,j ×PMP in,jn where c 0 i,j ,. .. c n i,j are predetermined arithmetical coefficients, and where i,j and n are integers.