For mature industries you can probably look at published government stats or aggregate the financials of leading companies. For new industries, it's a ballpark

For mature industries you can probably look at published government stats or aggregate the financials of leading companies. For new industries, it's a ballpark guess. For example, no one knew the market size of social media before Facebook. The best you can do is look at related industries like online search advertising and guess as to how much more people might pay to target ads based on a social media profile versus a Google search.