I second everything everyone said! The other piece I would point out is what biases/your POV could be present that could change the answer. For example, one market sizing question I was given was “how many college students are in the USA”. Now, there were a LOT of things I don't know: what % of 18 year olds enroll in college? What % of USA college students are international students? When the interviewer asked me what assumptions could be wrong, I said “I assumed x% of 18-22 year olds are in college, but this of course varies by state, city, socioeconomic status, etc. So while I guessed x% based on my experience/research, this could be a lot lower/higher for the country as a whole.”
I think as consulting firms are looking for people who are well rounded and diverse, take a look at your assumptions and say “did I estimate this because this is MY experience and could be different for others?"
This strategy of course isn't used for every market sizing question (no need to call out biases for number of golf balls that fit on an airplane) but as consulting firms look to be more inclusive, being introspective about assumptions can be good strategy