The candidate should think about a structure first and explain this to the interviewer before starting to make assumptions.
When the structure is clear, the candidate can start estimating by making his own assumptions without help from the interviewer.
This is a market sizing/estimation case so there is no right answer, but only a right approach.
Candidate should lay out his/her structure to estimate total market size. The request is for total market value so candidate is expected to give a monetary value.