A paragraph is a group of sentences with a single focus.
The topic sentence in the paragraph captures the central idea or focus of the paragraph.
The remaining sentences provide support and elaboration.
Within the paragraph there should be a unified focus.
If you find that your are writing short two and three sentence paragraphs it may well be that you are not developing your ideas and providing your reader with enough information on the topic.
In college writing assignments, paragraphs should be about 6 to 9 sentences. There should be no more than 3 paragraphs per page. If there are more, it likely means one or more paragraph is not adequately developed.
Never begin or end a paragraph with a quotation from a source. Paragraphs should begin in the student writer's voice and end in it as well.
If your paragraphs are running exceptionally long then it may be that you have multiple topics in the paragraph and you should consider breaking the long paragraph up into multiple paragraphs.
The following links provide additional guidance as you think about your own paragraph development techniques.