Keep the physical area as narrow as possible. For example, Lima but not Peru, Kuwait but not the Middle East, the eastern Amazon but not Brazil. The time period covered should be as short as possible. However, a decade or two should be the maximum. Focus the thesis on a narrow issue.
Every paper has a thesis, that is, a single major point that it is trying to make.
Your entire paper should be focused on this thesis, with little deviation. It is essential, then, that the thesis be well stated. A thesis might also suggest functional or structural relations. In all cases, the thesis provides the analytical focus of the paper. If you have any questions about your thesis, talk it over with the teacher.