Discuss why effective communicators should vary sentence and paragraph length in their writing.

Answers will vary. Sentences of short or average length are easy to read and preferred for communicating clearly. However, keeping all sentences short is undesirable, because the message might sound monotonous, unrealistic, or elementary. A 2-word sentence is acceptable; so is a 60-word sentence if it is clear.
Variety is just as desirable in paragraph length as it is in sentence length. A paragraph can be from one line in length to a dozen lines or more. The first and last paragraphs are usually short (one to four lines), and other paragraphs are no longer than six lines. A short first paragraph is more inviting to read than a long first paragraph, and a short last paragraph enables a writer to emphasize parting thoughts.