How do founders size a market (TAM/SAM) with AI assistance?

Use AI to do both directions fast: a top-down pass from industry reports and a bottom-up build (number of reachable customers times realistic price), then present both, since investors stress-test one against the other. AI is genuinely useful for finding data points, segmenting customer counts, and doing the arithmetic, but every number it produces needs a source you have personally opened, because sizing is where models hallucinate most confidently. The credible move in 2025-26 is a defensible bottom-up SOM, not a giant AI-generated TAM slide.

A quick orientation. The real value is below: resources worth your time, from people who've actually done it.