Do not freeze just because you do not know a real-world fact. State that you do not know the exact figure, propose a reasonable assumption, and explain the logic behind it.

The point of market sizing is not trivia recall. It is whether you can break an uncertain problem into manageable pieces and work consistently with your assumptions.

If the number is critical and the case likely expects interviewer data, ask whether an input is available. If not, choose a defensible range or benchmark and keep moving.

The phrase 'I'd assume X based on Y' saved me from overthinking this.

I used to think every unknown meant I failed. It's literally part of the exercise.