A paragraph presents a new topic or aspect of a topic. It should normally contain three elements:
1. A 'topic sentence', where the subject of the paragraph is introduced (e.g. 'Molière makes frequent use of verbal wit.').
2. Development and/or illustration (e.g. examples of Molière's verbal wit).
3. A concluding comment, summarising the content of the paragraph and linking the discussion back to the subject of the essay (so if the essay subject is 'Analyse Molière's comic technique in Le Tartuffe', you might conclude with 'This aspect of Molière's comic technique highlights the self-deception of characters such as Orgon; at the same time it delights the audience with its dazzling display of playful virtuosity.'.
Each paragraph should be linked to the next one. Link words such as 'but', 'nevertheless', 'in addition' are useful here; or you can repeat a word or expression or recapitulate an idea in the previous paragraph before moving on to the next point, e.g. 'Molière's verbal wit often derives from a "dialogue of the deaf".'.
From the above it is clear that a paragraph is unlikely to contain fewer than three sentences, and is likely to have more. However, it should not be so long as to make the reader lose track of the point. If you find yourself exceeding, say, half a page with one paragraph, see whether you can break it up into two or more different aspects of the topic.
Occasionally, however, a paragraph may be very short—perhaps even one short sentence—for a dramatic stylistic effect, most probably at the very beginning or end of an essay, or perhaps at its turning-point. For example, an essay outlining the failures of attempts to control global warming might conceivably end with a paragraph summing up the content of your discussion, and be followed by a single paragraph set off from the rest: 'L'avenir est sombre.' It's wise to use this device sparingly and be careful it doesn't seem over-the-top or fall flat.