Many great answers already around how to think about this. I would structure it around the use cases:
- Ethical - vegans who will switch to the shampoo for moral reasons
- Cosmetic - people who will switch to the shampoo as they are highly conscious of cosmetic products they use and ingredients
- Medical - people who HAVE to switch as vegan shampoos are the only ones suitable e.g. skin conditions like eczema
If I was the interviewer asking this question, I would like to see if you could identity unconventional markets like number 3 above with a bit of lateral thinking.
The general approach around estimating population, % of population for each, willingness to pay etc. would work as per all other market sizing questions.