In the beginning of the bipartite review on anthropometric parameters of ready-to-wear garments there are generalized requirements for the control dimensions choice among body measurements for one-, two-, or three-dimensional sizing purposes and that for psychophysically feasible inter-size indifference interval widths well-known as variable from the algebraic zero to the full range of measurements variability as in the case of a universal “one-size-fits-all”. An arithmetic progression type model iLn = (iL0 + id.n) is proposed for description of the key or control dimension scale partition to facilitate understanding various size sets and to validate logical design (algorithm) for computerized size identification. Depicting differences among Eastern European (former CMEA standards), German DOB-94, and EN 13402 chest/bust sizes respective graphs of the sizing step functions are superimposed thus quantitatively visualizing meanings of various menswear and womenswear size designations and estimating interchangeability of them. On the basis of the introduced prerequisites there are enabled further calculations of the optimal number of figure types, standardization of their dimensions, as well as size labelling and fit provability considerations to follow up in the second part of the paper.