How do you know if your franchise territory is big enough to support the type of income that you expect?

I would talk to other franchisees and ask them the size of their territories and try to get a sense of their earnings and then try to compare the territory you are being offered. Beyond size you have to look at the underlying demographics like income levels and other demographic characteristics of your target market - the franchisor should have this information.

Look at the franchisors financials in the UFOC and extract the royalty line divide that by the number of franchisees, then by the size of the territory i.e. number of people and you have your revenue per person as benchmark

Territories are so variable all you would be doing is making an educated guess. The best thing you can do is to try to find other franchisees who are in your area and try to get them to tell you their earnings numbers. Of course they are going to be reluctant to tell you if they view your franchise as a competitor