While bodies change over time and trends vary, most store-bought clothing comes in fixed sizes and styles and fails to adapt to these changes. Alterations can enable small changes to otherwise static garments, but these changes often require sewing and are non-reversible. We propose a modular approach to garment design that considers resizing, restyling, and reuse earlier in the design process. Our contributions include a compact set of modules and connectors that form the building blocks of modular garments, a method to decompose a garment into modules via integer linear programming, and a digital design tool that supports modular garment design and simulation. Our user evaluation suggests that our approach to modular design can support the creation of a wide range of garments and can help users transform them across sizes and styles while reusing the same building blocks.