Three qualities make a successful paragraph:

• unity

• coherence

• development

A paragraph should focus on one main idea (unity). Its parts should be clearly related (coherence). Finally, its main idea must be supported sufficiently with specifics or details (development).

Unity

A paragraph should have a topic sentence in order to state the main idea clearly. Each paragraph should usually begin with a topic sentence, and every single sentence in the paragraph should contribute or relate to the main idea.  If a sentence does not relate clearly to the topic sentence, you should throw it out.

Coherence

Not only do all the sentences of your paragraph need to belong together, but your reader must also be able to see how they fit together. A paragraph is coherent if its details fit together clearly in a way that readers can easily follow. Sentences must also relate to one another structurally. Coherence can be achieved by repeating key terms, organizing ideas, using parallel structure, pronouns, and transitions.

Development

In addition to being unified and coherent, a paragraph should hold the reader's interest and explore its topic fully using details and evidence. Illustrating a point with concrete examples can help to develop the main idea of a paragraph.

Length of Paragraphs

No rules are carved in stone tablets dictating how long a paragraph should be. However, for argumentative essays, a good rule of thumb is that, if your paragraph is shorter than five or six good, substantial sentences, then you should reexamine it to make sure that you've developed the ideas fully. Do not look at that rule of thumb, however, as hard and fast. It is simply a general guideline that may not fit some paragraphs. A paragraph should be long enough to do justice to the main idea of the paragraph. Sometimes a paragraph may be short; sometimes it will be long.  On the other hand, if your paragraph runs on to a page or longer, you should probably reexamine its coherence to make sure that you are sticking to only one main topic. Perhaps you can find subtopics that merit their own paragraphs. Think more about the unity, coherence, and development of a paragraph than the basic length.  If you are worried that a paragraph is too short, then it probably lacks sufficient development. If you are worried that a paragraph is too long, then you may have rambled on to topics other than the one stated in your topic sentence.