Determining the right target audience is probably the most important part of your marketing efforts, because it doesn't matter what you're saying if you're not saying it to the right people.
In this section of your marketing plan, go into as much detail as possible about who your market is. Describe your typical customer in detail. What is the age group, gender, education level, family size, income level, and geographic location. For business-to-business markets, make sure you include the industry type (or SIC/NAICS), company size, job titles/departments, annual revenue, and geographic areas. Have a general picture of who your market is, then back up that information with concrete numbers and statistics about the size of your market.
Determining the size of your market really requires that you already have a good profile of your typical customer. Once you know "whom" you're looking for, you need to take into consideration things like the aging of the population, and regional variations in income levels and education levels.
Your product offering will also require that you consider not only income levels, but also disposable and discretionary income levels. The former refers to income after taxes that is used to pay for daily living expenses, and the latter refers to income that is left over after those necessities are paid for and can be earmarked for luxuries.
Getting to this level of demographic data will probably require market research. Look for data about the regions where higher densities of these specific groups can be found. The Bureau of Labor Statistics and the U.S. Census Bureau have information about annual spending levels in the major categories of expenditures. You can also find data about age group concentrations in specific regions.