A topic is too broad when you find that you have too many different ideas or resources about that topic. While you want to start the writing process with as many ideas as possible, you will want to narrow your focus at some point so that you aren't attempting to do too much in one essay.
Here are ways to make your result list less in quantity, but still high in relevence:
A topic is too narrow if you can't find any information about it. Though student writers most often face the challenge of limiting a topic that is too broad, they occasionally have to recognize that they have chosen a topic that is too narrow or that they have narrowed a workable topic too much. If your topic is so narrowed and focused, it can become too academic or pedantic. If your topic is too narrow, try making it broader by asking yourself related questions.
Selecting a topic can be the most difficult part of doing research. Defining and refining your topic is an ongoing process. Be prepared to change the focus of your topic as you gather more information. How you select your topic will depend also on what your instructor has specified.
These are some general guidelines to follow:
Broad topic: anorexia --> Focused topic: What is the relationship between women's fashion magazines and anorexia?
Your research question should be opened ended, will guide you in further research, and will help you formulate your thesis.
(Westchester Community College, New York)