Market sizing is a critical component of business strategy and planning, but it is often executed incorrectly, leading to unrealistic expectations. The text emphasizes the importance of grounding market assessments in realism, especially for emerging products, by narrowing focus rather than casting too wide a net. This approach is demonstrated through a case study that moves beyond the traditional Total Addressable Market (TAM), Serviceable Attainable Market (SAM), and Serviceable Obtainable Market (SOM) model to a niche-focused strategy aimed at realistic and actionable outcomes. It advocates for understanding and defining a specific target market, as illustrated with a hypothetical financial app for SMEs in the EU, to avoid common pitfalls of overestimation and vague projections. The process involves detailed research and iterative refinement of assumptions and estimates, with an emphasis on periodic reassessment to adapt to changing market conditions and ensure strategic relevance.