Industry classification systems are created to promote data comparability. NAICS (current version) and SIC (older version) are the most widely used in the United States. You can search by classification codes to find information on industries or sectors in many books, databases, and government websites. However, NAICS and SIC focus on broader industry activities and may not have a specific code for niche/emerging industries. Many databases also have their own classification systems.
An industry association is established to advocate for the industry and associated companies. They collect and provide unique data/articles/reports. Sometimes these sources are on their websites for free.
To find the industry association websites:
Market research reports databases like IBISWorld, Mintel, and Passport have information on market size.
However, it's not always easy to find size information on a narrow/niche or emerging markets. When that happens, search for Business Source Premier and Nexis Uni to see if the info is mentioned in articles. Industry associations often provide estimates. Google is great at picking up news, blog posts, and press releases that may mention specific numbers.
You may also need to infer from broader market data and come up with your own estimates, esp. for new products/services that don't exist yet. Below are tutorials tutorials on how to size a market: