Peptide amphiphiles are molecules containing a peptide segment covalently bonded to a hydrophobic tail and are known to self-assemble in water into supramolecular nanostructures with shape diversity ranging from spheres to cylinders, twisted ribbons, belts, and tubes. Understanding the self-assembly mechanisms to control dimensions and shapes of the nanostructures remains a grand challenge. We report here on a systematic study of peptide amphiphiles containing valine glutamic acid dimeric repeats known to promote self-assembly into belt-like flat assemblies. We find that the lateral growth of the assemblies can be controlled in the range of 100 nm down to 10 nm as the number of dimeric repeats is increased from two to six. Using circular dichroism, the degree of beta-sheet twisting within the supramolecular assemblies was found to be directly proportional to the number of dimeric repeats in the PA molecule. Interestingly, as twisting increased, a threshold is reached where cylinders rather than flat assemblies become the dominant morphology. We also show that in the belt regime, the width of the nanostructures can be decreased by raising the pH to increase charge density and therefore electrostatic repulsion among glutamic acid residues. The control of size and shape of these nanostructures should affect their functions in biological signaling and drug delivery.