According to Merrian-Webster, "The longest word entered in most standard English dictionaries is Pneumonoultramicroscopicsilicovolcanoconiosis with 45 letters. Our definition is 'a lung disease caused by inhalation of very fine silicate or quartz dust.'" 1
Some people love BIG words! I won't question their motives, but they'll gladly throw out a few words you've never heard.
While preachers might know some big words (infralapsarianism, sublapsarianism, and supralapsarianism, to name a few), we don't do anyone any favors by showing off our large vocabulary in the pulpit.
I believe when it comes to communicating God's Word in teaching or preaching, simple is best. Use words that the people will understand.
JC Ryle in his book Simplicity in Preaching offers this advice:
Try to use in all your sermons, as far as you can—simple words. …Whatever you do, beware of what the poor shrewdly call "dictionary" words, that is, of words which are abstract, or scientific, or pedantic, or complicated, or vague, or very long. They may seem very fine, and sound very grand, but they are rarely of any use. The most powerful and forcible words, as a rule, are very short. 2
Warren & David Wiersbe offer this counsel in their little book The Elements of Preaching (I cannot recommend this book enough!)
Sometimes the message requires the use of theological terms, so take time to explain and illustrate them. But avoid “dropping in” words and phrases that your people do not understand. You will only encourage them to stop listening. “It must have been a great sermon,” said one parishioner; “I didn’t understand a word of it!” Impressed—but not helped. The time was wasted.
Avoid the temptation to show off your learning. Great teachers and preachers speak so as to be understood. This especially applies to references to Hebrew and Greek. Don’t give grammar lectures or drown the congregation in “cognates.” One minister based a funeral message on the meaning of the Greek word parakaleo, and used the word repeatedly. The mourners were not impressed—or comforted. 3
I still remember years ago when J. Vernon McGee talked about how he tried to always put the cookies on the bottom shelf where the children can get them. 4 That’s a good place to put them, by the way.
When preaching, it's good to remember this advice:
Preach to express, not to impress. Preach to be understood. It has well been said that the man who shoots above his target does not prove that he has better ammunition, but only that he cannot shoot. 5