How to estimate market's volume?

Here's one method I've used in the past to estimate market size:

1. Do keyword research. Collect enough related searches and long tail keywords that you feel those keywords are a good sample of who your audience is.
2. Use Google Trends to find the top geographical locations related to these keywords. Be specific! Not countries, but divided into the smallest possible zones that still have enough people for the market size you'd like to start with. Note: Trends only shows relative activity, not absolute. You'll also need to search for the population size of those regions.
3. On Trends, I also note the historical activity for those keywords. These trends can give me insights into the habits (trends) of those people. Related keywords outside of your project (from slightly related industries) can give more insight into how much they value a certain concept (e.g. data privacy in general versus the GDPR specifically) and the combined historical graphs can serve as a very rough estimate of how likely they are to feeling positive about adopting your new project.
4. I then add the population density for those geographical locations. (= reach more people with less effort)

With the above, you have a list of possible markets, the likeliness of adoption, population density, etc.

That has been a good starting point for me in the past. A potential next step is to reach out to someone in those markets manually (an influencer, someone with reach on social media or a blog, an existing related company) and use that channel to get first users onto your landing page / MVP to validate (= paying customers).