A persona is a decision-making tool, not a poster. One page, and it should let someone on the team settle an argument without asking you.
Useful contents: what they are trying to achieve, what they do today, where it hurts, how much they know about the domain, and the context they would use your product in — busy office, phone on a commute, shared machine.
If you are entering a new market you will not have existing customers to build from. Hypothesise, then test the hypothesis on people who match the profile. A persona built entirely from imagination is fiction with a photograph attached.