I'd suggest you turn things around and ask them to give you a proposal. I have seen too many disasters when people try to scope out a project without knowing how. Start with a basic scope. “I want a website”. Then sit down with the first potential developer and flesh it out. Move on to the next potential developer and let them give their input, suggest additions, poke holes in it, etc. When you are happy with the scope that have been developed and peer reviewed by a number of professionals put it out for bid.