A few years ago, I wrote a post titled “How Long Should a Paragraph Be?” which argued that various pronouncements that dictate paragraph length (expounded for the benefit of beginning writers, who presumably are aided by the introduction of a circumscribed formula for success in composition) should be ignored in favor of a commonsense approach to organizing paragraphs according to the ideas expressed within; the correct answer, I argued, is that a paragraph has to be long enough to reach its end, meaning that a paragraph can be as short or as long as is required for a writer to express an idea.
Did the preceding paragraph seem too long? It’s not especially lengthy, but if it exhausted you to read it, that’s because it consists of a single sentence that is more than a hundred words long. Although I am known to write long, complex sentences, that one, which I deliberately stretched out to an excessive extent, is an example of a statement that could use some reorganization.
How long should a sentence be? Like a paragraph, it should be long enough to reach its end, but, as with a paragraph, that objective should be balanced with aesthetic considerations. A sentence can consist of one word or be infinitely long, but what will serve the reader while expressing a complete thought?
Generally, it’s more productive to provide a sequence of sentences of naturally varied length than to dictate how many words one is permitted to use in a given sentence; a succession of sentences of equal or similar length will distract readers, as will a series with wildly divergent word counts. Take care not to repeatedly overwhelm sentences with multiple forms of parenthesis (interjecting words or phrases—or entire sentences, for that matter, using commas, parentheses, or dashes). The previous sentence includes the three basic forms, but note that, aside from a single semicolon, I have refrained from introducing anything more complicated into this paragraph.
Don’t overthink the issue, of course. Write naturally, but when revising your work, attend to sentence length and combine or separate sentences that seem too abrupt or unwieldy (unless that is the effect you want to create). If you want a ballpark figure, go with a range of twenty to twenty-five words as a benchmark, though average length will vary depending on the literacy of your readership.