If a paragraph is a group of sentences that take shape to form an idea, a long paragraph is simply an extension of this.
Paragraphs don’t really have one specific length, but it is accepted that in order for something to constitute a paragraph, it should have at least two sentences.
Defining something as a long paragraph will likely depend on the piece of writing you are compiling.
For example, if you are writing a thesis, a long paragraph could end up consisting of ten or more sentences. A long paragraph in a children’s book could be no more than three short sentences.
When writing a long paragraph, it is usually best to compare yours to something in a similar genre. If you are writing a novel, for example, pick up a similar book and take a look at the length of the average paragraph.
When in doubt, keep it short. Long paragraphs aren’t always helpful. In fact, they can be downright boring. In writing, as in life, less is often more.