The top-down approach is where many businesses start in determining the size of the market. With this technique, you’ll begin by researching the addressable or total market for your product or service.

Then you’ll need to form a reasonable estimate of your portion of that market, which becomes your available market. Thorough research of the market, your industry and competitors is the key to accurate estimation.

Most industries have reports that talk about market size held by existing leaders in the space. For example, Ibisworld and Gartner are two great places to start. They publish regular reports on consumer and business markets and they’re extremely accurate.

Here are more ways to figure out your market size so you can better define your targeted customer: http://www.fieldboom.com/blog/market-size/.