I would definitely be prepared to answer a market sizing question. It depends by office, but many offices give their interviews pretty much "free reign" over what questions they ask. When I interviewed at BCG roughly 3 years ago, I received a market sizing question in my final round from a senior partner.
In addition to Vlad's distinction of "standalone" or "part of a broader case", I would further distinguish two types of market sizings you should be ready for:
1) Top-down market sizings - e.g. "what is the value of the total hamburger market in the US?" This question you typically target my starting from total population, segmenting, etc..
2) Bottom-up market sizings (common at Bain) - e.g. "what is the revenue of a typical Tennis club in London?". This question you will struggle to approach top-down, so should approach from a capacity and utilisation approach.
Hope this is helpful!