I admit to being a "late adopter" to technology and, therefore, I learned only recently that my computer has a "readability index" in Word. To make use of this valuable tool, all you have to do is enable it. (Sorry, you'll have to ask a techie.) Then, when you spellcheck any document, your computer will give you a readability report after it has told you to correct any spelling errors.
The readability index is based on the Flesch readability studies. The index measures the length of your words, sentences, and paragraphs and your use of passive voice. The tool suggests that most people read at a level between 8th and 11th grade and suggests, therefore, that if you want to reach most audiences, you need to use short words, short sentences and short paragraphs. Also, because active voice is more direct, the index suggests that you prefer active voice over passive.
I recently copied and pasted into Word Barack Obama's inauguration speech and his first report to Congress, just to see how he scored. These were speeches, of course, but they began as written documents, very few people speak extemporaneously these days, especially if they are presidents and paying talented speech writers.
So, how did Barack score on readability (think "understandability")? He scored mid-8th grade in the inaugural address and mid-9th grade in the speech to Congress.
You might say that he should have written to the mid-3rd grade level for Congress. Those folks have not been accused of being the brightest candles on the cake. In contrast to many elected officials, Winston Churchill understood the need to use plain language. I scored his famous "we will fight on the seas..." speech, the one he gave during WWII, and it scored mid-3rd grade level. Mid-3rd grade level! He certainly knew his audience! Both Obama and Churchill are known as great intellectuals, yet both chose to address his audience on a level that he was certain would communicate his message.
Great orators of English know that they need to use strong Anglo-Saxon words (the short kind) and short, crisp sentences. They know that they should avoid words from the Latin and Greek . You might enjoy writing to a co-worker and saying, "You are sui generis." Or, you might want to say to a relative, "Give up on being a omphaloskepsist," just to show off your learning, but you won't make your point as well as if you had said, "You are one of a kind!" and "Get your nose out of your navel."
The long words are for the academics and bureaucrats. Many academics write to impress each other and many bureaucrats write to conceal, not reveal. Long words (Latin and Greek words) help them. I encourage you to study the Greek and Latin origins of English to better understand the language, but avoid using the words, especially in business writing, in favor of their cousins, the Anglo-Saxons.
If you're not sure how your writing (and speaking) score, use the readability tool in Word. It's fun and revealing!