How do I decide on the scope of my essay?

This may sound funny, but the scope of your paper needs to be the right size. That is, your topic can't be too big or too small. Like in "Goldilocks and the Three Bears," the topic must be "just right."
When you do your research, you can get a good idea of the scope of your topic by looking at what's available. If there's an overabundance of material, your topic is probably too big. For example, it would be hard to summarize the history of rock and roll in a three-page paper because the scope of the topic is so large. One the other hand, writing about the use of the sitar (an instrument) in rock and roll probably isn't big enough. Writing about Eastern musical influences on rock, though, might be just right.