Other than global market size, depending on the exact question, you can try to compare in other ways that come a bit closer to your life. E.g.
- Compare to another industry you roughly know in the country, and see intuitively whether it makes sense (should the estimated market be bigger or smaller than this benchmark industry?)
- If the estimate is for B2C business, compare that number to the population and check the per capita number, does it make sense?
Try to see if you can use some of the common sense understanding in life to sense check. Even if you couldn't find a good benchmark, don't worry too much. You are tested more for whether you can think logically in a systematic approach. If you get that accomplished, but just make a wrong assumption because you are not familiar with certain industry, you would still get the most credit.