Here's one account by John Davenport in Aphrodisiacs and Love Stimulants (Lyle Stuart, 1966):

A remarkable instance is given by [Sir Edward] Home. It occurred in a negress who was purchased by General Melville, in the island of Dominica, in the West Indies, about the year 1744. She was of the Mandango nation, twenty-four years of age, her breasts were very flat, she had a rough voice and a masculine countenance. The clitoris was two inches long [5 cm], and in some instances resembled a common-sized thumb. When viewed at some distance the end appeared round and of a red color, but upon closer examination was found to be more pointed than that of a penis, and having neither prepuce nor perforation; when handled it became erected, and was in that state fully 3 inches long and much thicker than before…The other parts of the female organs were found to be in a natural state.

One wonders who was doing the "handling" here, but the passage doesn't say. In 1824 another doctor, identified as "Otto at Breslau," reported seeing an African women with a clitoris 4½ inches long and 1½ inches diameter (11.4 by 3.8 cm), said to form a complete covering of the vaginal orifice. Another doctor, Bainbridge, related the case of enlarged clitoris in a woman of thirty-two whose organ was five inches in length (13 cm) and "about the diameter of a quiescent penis."

In what may be the ultimate expression of medial one-uppance, an 1813 French medical encyclopedia even goes as far as to claim existence of a monstrous twelve-inch specimen (31 cm!), said to be like "the neck of a goose." This seems preposterous: come on, most guys aren't even that hung.

A more credible claim would be from the 18^th century Swiss biologist Albrecht von Haller, who is said to have come across a woman with a monstrous clitoris measuring no less than seven inches long (18 cm).

As an aside, it's kind of interesting to read the sort of goofy speculation some male chroniclers make on large clitorises. One claim, shared by Robert L. Dickinson, he of the largest penis fame, and Alfred Kinsey, is that they were strictly "utilized as [a] copulator by lesbians." Well, maybe. But don't you think you'd have to own the equipment before you make any claims on how to operate it?