Addition of decanol to the system sodium dodecyl (lauryl) sulfate (SLS)/water leads to the appearance of a nematic domain not present in the binary system. We investigate the structure of the hexagonal phase (H) of the ternary system for a fixed water:SLS molar ratio of 39.4. The decanol:SLS molar ratio was varied in the interval 0.12-0.37, inducing the phase sequence isotropic (I)-H-nematic cylindrical (N-c). Analysis of the obtained hexagonal parameter as a function of (SLS + decanol) volume concentration shows that the H phase in the ternary system shows two dimension swelling, as expected for infinite (or flexible) objects. On the other hand, the H phase in the binary system shows three dimension swelling, as expected for rigid finite objects. The micellar radius, necessary for the above analysis, was determined from electron density maps, obtained from observed diffraction intensities. A "crossover" from the finite rigid model to the infinite model occurs in the ternary system, due to the effect of decanol, which promotes micellar growth, changes the characteristics of the H phase, and induces the entrance on the N-c phase. The possible reasons for the nontrivial I-H-N-c sequence are discussed on basis of existing theories for self-assembly systems of rigid and flexible rods and for the elastic bending energy of mixed micelles.