Building a market size from the bottom up
A defensible market size is a count of customers multiplied by what they can be charged. Anything else is a citation.
Start from a count
Name the unit that buys. Companies of a given size in a given sector, clinics of a given type, vehicles in a given fleet class. Then count them from a source an investor can check, and state the source.
The count is the part of the calculation that is verifiable, so it carries the credibility of the whole number.
Then price them
Use the contract value you actually charge today, not the one in the pricing page and not the one you intend to charge at scale. If expansion is part of the thesis, show it as a separate layer with its own evidence.
Where the price is set by a budget line the customer already spends, say which line and what it currently costs them. Displacement is easier to underwrite than creation.
Constrain the reachable part
The addressable number is not the reachable one. Apply the constraints that actually bind: geography you can sell into, regulatory approval you hold, integrations that exist, sales capacity you can fund from this round.
A smaller number arrived at honestly is more persuasive than a large one arrived at by division, and it makes the round size look deliberate rather than arbitrary.