Your Total Available Market (TAM) size are:
NOT Top Down,
NOT the size of the problem,
NOT the size of the market today,
ARE the number of customer multiply by your price
Estimated size should:
be meaningful,
somewhat achievable with the current business model & strategy.
It should target your customer, not someone else's.
Market size estimates is an aspiration and roadmap for a start-up to earn this market share.