The market sizing playbook every top investor wants to see

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Author

Michael Morgenstern
September 16, 2025

Happy Tuesday Brex Community,

Market sizing isn’t about proving that the world is big. It’s about proving that you know exactly which slice of it belongs to you. A credible founder starts from the ground up: Who are the customers, how many exist, what do they pay, and how much of that pie can you realistically capture? The goal is to connect those dots into a clear line to $100M in gross profit, and eventually $500M. These are my thresholds that signal “venture scale” and “fund-returning.” That means segmentation isn’t optional. It is the whole point. By showing investors you’ve thought about your market in terms of buyer personas, willingness to pay, and adoption patterns, you turn a generic TAM slide into an investible thesis.

The deeper power of market sizing is that it tells a story of scale. A thoughtful model doesn’t stop at current spend. It illustrates how a wedge grows into an expanding segment. How contract values rise as you move up-market. And how tailwinds reshape the opportunity while you’re executing. Good founders don’t inflate numbers. They anchor them in reality, then layer in the forces that compound growth over time. That’s what convinces investors. Please no more trillion-dollar reports pulled from the internet, but rather narratives that makes the destination both ambitious and believable.

Growing together, Michael Morgenstern 

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