If you choose a project type, this will add additional fields at the Project Level that you can use to filter. The Contract ID is one of the fields you can choose on the Search Options link to filter your search.
What we wanted to do is get a count of all documents in all contract workspaces regardless of the number of workspaces. Here is how you do this using Search (instead of Reports).
Start on the Contracts Dashboard. Select Search, then, Any Document. This opens the Search Filter screen. Make sure Project Type of Contract Workspace (Procurement) is selected.
Click on Search Options and select the Ignore Max Search Limits option under Advanced.
Click on the Search button. The search will retrieve a list of all such documents and a total number.
We needed the information to help manage a data migration project.