Note that the Old Testament uses over 4,000 more different words than the New Testament. The difference is partly due to the fact that in the O.T. many more places and persons are named. Also, some of the books in the New Testament are very similar, such as Mark and Matthew. This does not, however, refute the assumption that the style of the Old Testament is simply much richer.
The shortest words in the KJV are a, I and O. They occur in all three parts of the Bible.
Longest word in the Old Testament:
longest: Mahershalalhashbaz, 18 characters. Maher-shalal-hash-baz was a son of the prophet Isaiah. The name means "speed the spoil, hasten the plunder", and was given at the moment that the king of Assyria was on his way and would soon rob the Syrians (in Damascus) and the Israelites (in Samaria). Isaiah prophetised that that would happen before the boy Mahershalalhashbaz would be able to cry 'mommy' or 'daddy' — let alone pronounce his own name.
The shortest verse in the Old Testament is 1 Chronicles 1:25, where 17 characters make "Eber, Peleg, Reu,". The Old Testament contains many such rather boring listings with names. If we exclude them, the top 10 gets more interesting.
Esther 8:9, 528 characters: Then were the king's scribes called at that time in the third month, that is, the month Sivan, on the three and twentieth day thereof; and it was written according to all that Mordecai commanded unto the Jews, and to the lieutenants, and the deputies and rulers of the provinces which are from India unto Ethiopia, an hundred twenty and seven provinces, unto every province according to the writing thereof, and unto every people after their language, and to the Jews according to their writing, and according to their language.
2 Kings 16:15: And king Ahaz commanded Urijah the priest, [..]