in reply to Starting a Large Project
If by "scope" you mean mental picture for how the system will fall together, I suggest you take one step back, and focus first on how the system will be used, and then get real users to validate your usage model. Otherwise, you risk doing a great job of building the wrong thing.
One quick way to develop a usage model is to use storyboarding, where you use pictures (e.g., prototyped screens shots) to walk users through a typical use of the system. This gives users something tangible to give feedback on.
If you want to get more formal, you can capture usage scenarios in "Use Cases". (Most books on the UML or the Rational Unified Process will tell you more than you need to know about Use Cases). I've used Use Cases on big projects, and have seen a good payoff.
Then, once both you and your target users are satisfied that you're building the right thing, use some of the excellent advice in the posts above.