This is the #1 biggest pitfall in the research process. If you pick a topic that is too big, you will not only have trouble selecting what to include from a huge selection of material available, you will probably leave out some critical information that will make it apparent (especially to your professor) that you don't really know what you are talking about.
If, on the other hand, you pick a topic that is too narrow, you won't find enough to write about and end up repeating yourself to fill 6 pages (which doesn't go over very well with professors either, by the way).
The Cambridge Rindge & Latin School (CRLS), from which this information has been excerpted, has developed a terrific tip sheet. This tip sheet explains clearly how to focus your topic. Also, there are links to other tips that you will find useful. Click here for the Focusing a Topic tip sheet.