The self-assembly of amino-undecyl-triethoxysilane (AUT) as micelles in water is considered. The behavior of acid/AUT systems is governed by a complete proton transfer from the acid to the amine, leading to the formation of an ammonium headgroup. This moiety is responsible for the bending of the interface between the organic core of the micelles and the surrounding water. By playing with the size of the acid used as curvature agent, the amphiphilic behavior of the organosilane molecule may be adjusted. We follow the aggregation as the curvature agent size increases. This approach constitutes an efficient and original method in order to tune the nanostructure of highly functionalized silica at the early stage of the elaboration. Small-angle X-ray scattering, wet scanning transmission electron microscopy, dynamic light scattering, and complementary characterization techniques indicate that hybrid organic-inorganic planar objects and vesicles are obtained for smaller curvature agents. Increasing the size of the curvature agent results in a transition of the aggregation geometry from vesicles to cylindrical direct micelles, finally leading to nanofibers organized in a 2D hexagonal network resembling a "reverse MCM-41 structure". A geometrical molecular self-assembly model is finally proposed, considering the dimensions of the surfactant tail and those of the head groups.