The ideal paragraph length depends not only on the kind of writing you're doing and the style and tone you adopt, but also on the format and medium in which you're writing. It's as much a visual as a verbal question. Different kinds of writing demand different average paragraph lengths. The classic tabloid newspaper style is to make each sentence a paragraph, and to keep each sentence short. This creates a telegraphic, immediate style well suited to news stories, and well suited to the narrow columns of tabloids.
Paragraphs average fewer than a dozen words.
Paragraphs in the New York Times, for instance, tend to be more in the range of 50-60 words. This is the style of writing usually taught in schools. Each paragraph, ideally, deals with a single thought. The thought is developed in succeeding sentences. Distinctions and connections are made, qualifications are introduced, and examples given.