There is no fixed answer to this. It depends on a case-by-case basis. At the end of the day you must understand the objective of market sizing questions - to gauge your sense and logic.

You could have a market-sizing exercise with literally no numbers given - which means you are also being tested on your logic while taking assumptions. Many people run the mistake of taking “shot-in-the-dark” assumptions - without basing them in some kind of logic or experience.

Alternatively, you could also have some of the numbers given - such as the one you mention. But it is entirely up to the interviewer on how it will be.

Hi, usually you are supposed to estimate every number you are able to. However, there are no written rules about it, the interview is a process of continuous alignment and problem solving with the interviewer, so I recommend running the case with her

Best,
Antonello