Low-dimensional molecular J-aggregates are extensively studied during past decades. The interest is caused by the unique nonlinear optical properties of these systems. The J-aggregates possess extremely high value of nonlinear susceptibility of third order with picosecond relaxation time. That makes this materials perspective for photonics and optical communication. The unique properties of the J-aggregates are explained by the delocalization of an excitation in an aggregate. However, the nature of the extremely high nonlinear susceptibility and its relation with excitation delocalization were not experimentally tested yet. Molecular aggregates of pseudoisocyanine with various width of a distinctive absorption line were obtained in thin (15nm) films on glass substrates (30x30 mm). These films possess a giant optical nonlinearity of third order and high photochemical stability at the room conditions. Exciton delocalization length was derived from absorption line width that is attributed to a mixing of lowest exciton band states. Nonlinear optical susceptibility of third order within resonance absorption band was measured by Z-scan and Pump-Probe techniques. Experimental results reveal a scaling dependence of the nonlinear optical susceptibility versus the exciton delocalization length, with a power factor about 2.3. This result coincides with the theoretical prediction for the phenomenon of size enhancement. High optical quality of the obtained film surfaces makes them perspective for applications as two-dimensional optical switches.