What is market sizing?

Market sizing is estimating how much revenue a market represents, usually expressed as the TAM/SAM/SOM cascade: total addressable market, the serviceable slice your product can actually reach, and the share you can realistically obtain. Top-down sizing scales an analyst figure; bottom-up sizing multiplies real buyers by real prices — and is almost always more credible.

The number itself matters less than what it forces you to define: who counts as a buyer, at what price, against which alternatives. A sizing exercise that ignores the competitors already serving the market overstates the opportunity by exactly their share.