A paragraph is a logical grouping of sentences. A paragraph should be used to develop one point and no more, but it shouldn’t be so long as to confuse a reader. Long paragraphs are difficult to understand so if you feel that your point needs more than five or six sentences to develop it then you should try and find a logical place to split the paragraph. If you are talking about a different point at the end of a paragraph than at the start this is a sign that the paragraph should be split.
Conversely, many short paragraphs can become boring for the reader. There is a tendency in modern day web-copy to “dumb down” the copy by having one sentence per paragraph. In our opinion this completely negates the point of paragraph as a logical grouping of sentences related to a particular point and should be avoided.