Shoe sizing sounds simple. It isn't. Feet are asymmetrical. They change shape over time. The most important measurement — girth, the circumference of the foot — is almost never factored into standard sizing charts. And existing digital sizing tools had failed not because the measurement was wrong, but because they didn't know what to do with it. We started with user research in two parts: targeted surveys to understand shopping behaviours and how people think about shoe purchases, and in-depth interviews with stakeholders, technologists, and footwear specialists to map the domain from the inside. From that, we built a service ecosystem canvas and identified the key touchpoints and user flows for the RSC Android application. Three design principles guided the app: a simple, guided measurement experience that makes your foot data an invisible marker for everything that follows; personalisation and discovery that shows you only shoes that will actually fit; and a storytelling layer that creates awareness of why this problem matters — both for individual health and for the environment. The design moved quickly into high-fidelity prototyping in Figma, used to test with both the client and the development team before a single line of production code was written.