There is no set length for a paragraph. It is possible, however, to have your paragraphs too long or too short. There are some guiding principles that will help you to get your paragraphs right.
The entire paragraph should concern itself with a single focus. If it begins with a one focus or major point of discussion, it should not end with another or wander within different ideas. This is one reason why paragraphs can become over-long. More will be said later about maintaining focus in your writing.
A paragraph should usually begin with an introductory sentence, which sets out the subject of that paragraph. The remainder of the paragraph should go on to explain and 'unpack' that initial sentence. If you find that you are writing about something different from your initial sentence, your paragraph is probably too long and your focus has wandered.
If you find that your paragraphs are too long:
Consider splitting a single long paragraph into two shorter ones. It is perfectly acceptable to begin a paragraph with a sentence connecting it to the previous paragraph.
Try to organise your writing so that your ideas are developed logically and sequentially. If you find that a paragraph contains more than one idea, you may need to reorganise your essay so that your ideas are developed more logically.
Look at the other paragraphs in your essay. Paragraphs should all be of roughly similar length. If you find that you have one or two paragraphs that are much longer than all the others, read them carefully and try to find out why.
If a paragraph is too short, it may be because the initial idea has not been developed sufficiently. To some extent, the level of development is dependent on the writer's purpose and the overall length of the essay. However, you should beware of paragraphs of only two or three sentences. Read them carefully and consider if your idea has been sufficiently developed.
If you think that an idea requires further development, consider some of the following strategies:
Use examples and illustrations
Cite data (facts, statistics, evidence, details, and others)
Examine testimony (what other people say such as quotes and paraphrases)
Use an anecdote or story
Define terms in the paragraph
Compare and contrast
Evaluate causes and reasons
Examine effects and consequences.