The MLA style guide dictates that you should keep your citation to 40 words or less, but that's purely for stylistic reasons, not legal reasons. Anything longer should be placed in a block quote, not a citation. (Source)
When citing a paragraph of a written work on a website, i.e. for an article, the general rule is to only cite what sentences are relevant to what you're discussing, and keep it as brief as possible. So for example, if I wanted to discuss a short paragraph of a novel in an article about word choice, I would only cite the parts that contain word choices I want to specifically address in my article. This aids in both user readability and keeping the article short and concise.
Citing long paragraphs and swathes of text, however, is generally discouraged, but not specifically illegal. There is no law that explicitly states how long a citation can be without verging into infringement territory. But as long as you keep it around 40 words, you should be fine.
(Disclaimer: I am not a lawyer, only somebody who has studied and referenced copyright law in the past for academic and literary purposes.)