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' Census.gov website. Use whichever dataset is most up to date for your defined area. This is not generally intuitive or user friendly, so ask for help if you need it.
This database provides access to market research reports covering US and international marketplaces. Each report combines data & analysis of the competitive landscape, market-share analysis and consumer profiles. Complex demographic issues are broken into easy-to-understand sections, explaining consumer behavior and demonstrating the structure of the market.