The variation of the nonlinear optical electronic polarizabilities of eight families of organic oligomers with molecular chain length is investigated using the coupled electronic oscillator (CEO) approach. Comparison of the saturation curves with contour plots of the first- and third-order density-matrix response shows a strong correlation between the magnitude of the saturation size, L-s, and the antidiagonal size of the density matrix, L-p, which constitutes a characteristic exciton size responsible for optical coherence. For the majority of oligomers investigated, L-s is linearly related to L-p.