Agree: Early-stage investing is rarely about accurately sizing a market. And nothing can be boiled satisfactorily down to an Excel model.
Disagree: But you still have to do it. For yourselves, if not for VCs. Presumably, VCs want to see that you've thought long and hard about the market opportunity, about the competition, and about your marketing plan and business model. And you should have. Your market sizing will never be pitch-perfect, but you can't just wave your hands in an investor meeting and declare that the entire exercise is bunk.
I would argue that, if you're being asked to "dig deeper" into market sizing, or even if you're being questioned at all about the market size in your meetings, it's really a proxy question for something else. Maybe they find the idea narrow, and they want to be convinced it can broaden to other segments. Maybe they think you're being insufficiently ambitious. Maybe it's the opposite; they think you're being wildly naive. Whatever the case, "What's the size of this market?" is a question their analysts can probably guesstimate in a few days' worth of Excel monkeying. That's not really what they're getting at. What they really want to know is how you think about the business side of your startup (or indeed, whether you do).