Write it together.
Grab a cup of coffee together, in person, or each on Skype and brainstorm over several hours. Talk talk talk.
If your developer has any experience, his inner architect will come out and he will know what needs to be done.
Ask him to help you write a draft, go over it together with Google docs, each highlighting in a different color, and presto. There is your specification.
If the developer is a freelancer, compensate him for his design and time; 4x the hourly development rates is fair for arch/design.
