Be elastic & be specific. Both are necessary. Narrow down your topic when your topic is too broad or when you find too much information. Expand your topic when you do not find enough information. You can also use the following techniques to help you narrow down or expand your searches.
Look for subject words on your topic.
Subject words are "controlled vocabulary" -- words and phrases -- that are used to categorize / organize informations resources (books, articles, and other media) so researchers can find them easily: Grouped under a "subject" regardless of what synonyms, variant spellings, and homographs authors/editors may have used. Researchers can't invent subject words but they can discover the subject word(s) for a topic in the description of a book, an article, or other information sources. ~ paraphrased from this source.