Have you ever watched children test their boundaries?
They keep pushing outward until someone or something makes it clear that they should not go any further.
Setting scope requires the same process of pushing. Keep asking questions that take you further away from the core problem until you reach agreement that you have gone too far and need to pull back. Then write down why you chose that boundary so that your decision can be reviewed and used as input to other project phases and projects.

A well-defined and communicated scope statement is one of the primary methods of managing a client's often-changing expectations of what they will ultimately receive from a project. This is developed during the proposal phase of the project and is included within the more comprehensive scope of work document. There are really two types of scope statements, each serving a different purpose in clarifying scope and expectations of a project and its deliverables.

1. The first is a product or deliverable scope statement;
2. the second is a project scope statement.

The deliverable and project scope statements should be developed in conjunction with the site-survey process and verified during the subsequent field verification audit process. Both of these are opportune times to visually communicate to clients what will and what not will be delivered by the integrator, as well as and who will and who will not be responsible for the implementation effort.