Via Rob MacDougal, Roy Peter Clark quotes Gary Provost in his book Writing Tools: 50 Essential Strategies for Every Writer:
This sentence has five words. Here are five more words. Five-word sentences are fine. But several together become monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It’s like a stuck record. The ear demands some variety. Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasant rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes, when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals–sounds that say listen to this, it is important.
This is why my sentences nearly always contain between 27 and 54 words. For variation.
And as “Weird Al” Yankovic taught us many years ago, songs should have more than five words too. In their case, 6 is the magic number: