The Project Complexity is : The extent to which a project, or one of its components, involves a large number of parts, and/or a large number of people, to be coordinated and/or interfaced. In project management, project complexity is typically a reflection of the number of work packages involved and the number of different people required to carry them out.
Note: The degree of project complexity may well have a bearing on the most appropriate style of management to be applied in the conduct of the project.

The Source :

http://www.nwcg.gov/teams/pmo/products/pmglossary/Proj.htm