A market sizing question asks how big something is: how many bike helmets are sold each year, or what a country spends on dog food. Nobody expects the true number. The interviewer is checking that your logic holds and your assumptions are sensible.
- Clarify the scope: which country, which year, units or dollars.
- Draw the driver tree before any math. Start from a population you know, narrow it with shares, then multiply by how often and how much.
- Pick round assumptions and say why: "about a third of households have a dog" is fine.
- Multiply step by step, rounding as you go.
- Sanity-check the answer against something you know.
In the builder, each box is a driver. Boxes multiply unless you switch one to divide. Facts the interviewer gives you’re marked Given.