You can estimate your target demographics by using Mintel as described in the researching demand tab.
To determine the size of the population that fits your target customer base, use the US Census decennial census or their more frequent Community Surveys. Use the most popular tables which allow you to find information about the population by age, gender, educational level, income, etc. for any defined geographic area. The tools are in the Census' American FactFinder website. Use whichever dataset is most up to date for your defined area.