It should be the maximum number of people that you can convincingly argue are potential customers. It sounds like you need to do more research.
If you say 65 million people in the US, you should be prepared to answer a question like: "Out of those 65 million people, how many have a spouse that doesn't work full time who fills this role?"
Agreed. Should definitely be more niche - also need to take into account internet usage and demographics such as disposable income. If someone works full time and makes less than $20K, they're probably not in your target market.
