Determining the market size for your research/potential product is absolutely crucial in order to succesfully plan the scope of your work and is necessary when applying for grant or VC funding. However, this can sometimes be a frustrating, confusing, and time-consuming process. What follows is a very broad overview that should help get you started, but it is recommended to reach out to a librarian if you get lost in the weeds.
Though the details will differ, the following are the basic to questions to consider when determining market size: