Hey,
Let me start by congratulating you on your upcoming role at MBB.
Regarding your question and/or any market estimation in general (whenever you can't find any report with the data are already off-the-shelf somewhere else), there is two main options to go: top-down or bottom-up. Assuming it is a too fragmented market, it would be difficult to go into a bottom-up, so most likely you will need to find a proxy/approach to reach a value from a top-down perspective - which is just the same problem solving method you need to use in consulting interviews asking for a market sizing estimation.
That said, if I was you I wouldn't be worried about this type of issues... as you shouldn't be expecting the clients to just pop up in the team room and ask such questions out of nowhere, so you will have plenty of time to problem solve it (with the help of your team)
Best
Bruno