When yo are in the initial stages of an idea, and when the market really doesn’t exist or there is little data about its size and growth, I have found it helpful to list all the markets you could go after, then plot them on a Suffering vs. Willingness to Pay matrix.
In this example with one of my clients, we realized that she was better off focusing her efforts on creating Jewelry for weddings (because brides want their bridesmaids to have matching dresses and jewelry) and jewelry that husbands can give to their wife (because husbands want to give something special but have a hard time deciding what to give). She could go after many types of clients, but there is more suffering for lack of a good existing solution and more willingness to pay with those two particular customer types.