Controlled peptide assembly offers significant promise to develop synthetic supramolecular nanostructures to display material and biological properties that mimic protein assemblies in nature. Despite the progress in forming peptide nanostructures of various morphology, there exists a distinct gap between natural and synthetic assembly systems in terms of size control. Constructing nanostructures with a narrow size distribution that can be tuned over a wide range of length-scales is essential for applications that require precise spacing between objects. This approach provides the opportunity to correlate materials and biological properties of interest with assembly size. In this review, we discuss representative endeavors over the past two decades for design of size-controllable peptide nanostructures using tunable building blocks. Other mechanisms for size control, e.g., molecular frustration, template-directed peptide assembly, and multi-component peptide co-assembly, will also be discussed. We also demonstrate the applicable scopes of these strategies and suggest potential future avenues for scientific advances in this field.