A paragraph can be as long as it has to be. A paragraph is meant to contain an idea. It can be 20 sentences long or it can be 6 sentences long.
Even if you were to define that a paragraph is 6 sentences long, then comes the question, how long should a sentence be? 10 words? Or 25 words? And then comes how many letters the words should be on average. 2 letters per word on average? Or 7?
So how long a paragraph can be is all relative...
