To be specific, you have to be smart, or even crafty. For instance, you may examine annual reports from corporations that may include interesting facts about their target segment within the description of how their products are doing. Another trick is to use tools that are primarily made for other purposes. For instance, by trying to promote something on Facebook (a post, a page, or an app), you can define an ad campaign, then Facebook provides you with tools to target your population (gender, age, device [iPhone or Android], and interests) and displays information about the potentially reached population, which in turn gives you an idea of its size. You do not need to actually launch the campaign and pay for this.