A paragraph can be as short as a word or as long as 250 words or more. It may consist of just one sentence or many sentences. The literary essay may be composed of long paragraphs. J.C. Tressler and Claude E. Lewis say that the average length of a paragraph is 100 to 150 words for ordinary writing; about 60 for business letters; and about 75 for newspaper reports .
According to some plain English experts, a plain paragraph is no more than four to six sentences long. They also feel that a paragraph of more than four or five lines can make a text less readable .
Martin Cutts dismisses the so-called rule ‘never write a one-sentence paragraph’ as a myth. He writes: “If you can say what you want to say in a single sentence that lacks a direct connection with any other sentence, just stop there and go on to a new paragraph. There’s no rule against it. A paragraph can be a single sentence, whether long, short, or middling.” For online texts, he recommends short paragraphs of 40 to 50 words. 
Since text may be set in different kinds of type, Cameron S. Foote offers some thumb rules: about 10 words per line for serif text; and about eight words per line for sans serif text. 
Says Hildegard Korger: “In an English text eight to ten words or forty to fifty characters per line seem to produce the right length for one line.” 
The foregoing paragraphs have supplied us with enough data for the following discussion on the length of the paragraph. And we have seen that a paragraph may be measured in sentences and lines.
In terms of sentences, a plain paragraph has four to six sentences. Since the average sentence length is about 17 words, a plain paragraph has 68 to 102 words. But online texts need short paragraphs of 40 to 50 words, translating to three sentences or less.
In terms of lines, a plain paragraph has four or five lines. Each line has eight to ten words or 40 to 50 characters. A simple arithmetic division confirms that the average length of a word is five characters. This independently confirms the letter-word part of the equation 1 x L (letters) = 3 x S (syllables) = 5 x W (words), discussed in my article titled ‘Syllable, Word And Sentence Length’ at Readability Monitor. If we assume that the syllabic part of the equation is also true, then each line has about 13 to 17 syllables.
For those who need only the guidelines, here they are:
* The average line length is 40 to 50 characters; 13 to 17 syllables; and eight to 10 words.
* The plain paragraph is four to six sentences; four or five lines; and 68 to 102 words. But the online paragraph must be short consisting of 50 words or less; and three sentences or less.
 Mastering Effective English: J.C. Tressler and Claude E. Lewis, The Copp Clark Publishing Co. Limited, Vancouver, 1950
 Plain Language In Plain English: Edited by Cheryl Stephens, Plain Language Wizardry, Vancouver, 2010
 Oxford Guide To Plain English: Martin Cutts, 3rd edition, Oxford University Press, 2009
 The Fourth Medium: Cameron S. Foote, Dow-Jones Irwin, Illinois, 1986
 Handbook Of Type And Lettering: Hildegard Korger (English translation by Ingrid Li), Design Press, New York, 1992