The worst thing you can do in that case - rush into calculating the market sie without identifying the future of the product.
First of all, I would ask the interviewer if there is any specific segment that we would like to target. (e.g. it will be an exclusive drug for the rich people, or it will be the mass segment).
If there is no clarity on this, I will approach with a broader structure:
Analyze our product:
- The value proposition of a product (Core / non-core features),
- Limitations, side effects, negative consequences
- Different use cases for the product (Business / private / military, etc)
- Key capabilities to compete (Patent, distribution, etc)
Define the target segment and pricing:
Calculate the market size
- Size of the segment / segments
- Expected usage frequency
- (e.g. if you identified that it will be the military use of the drug - then you need to understand the size of the army and the usage frequency)