Shorter paragraphs, with two to three sentences, are often used to introduce a new topic or to present a main idea. They can be effective for grabbing the reader's attention and setting the stage for the rest of the paragraph. Longer paragraphs, with five or more sentences, are often used to develop and elaborate on a particular idea or point. They provide an opportunity for the writer to provide more details, examples, or evidence to support their argument.
Ultimately, the number of sentences in a paragraph is a matter of style and should be determined based on the purpose of the writing and the desired impact on the reader.