The tiniest word that is recognized as a word — in the sense that there's an OED entry — is "fancyette."

It's a noun made out of the noun "fancy" — meaning fantasy or figment of imagination — and the ending "-ette" — meaning a small version of something. The OED defines it as "A little fancy" and says it's "Apparently an isolated use." The one example of the use — perhaps the only example — is:

a1834 S. T. Coleridge Marginalia in Blackwood's Edinb. Mag. (1882) Jan. 125 [Two Fancyettes, as Coleridge names them, at the end of a volume of Fichte].

So the poet Samuel Taylor Coleridge wrote marginalia in a magazine 140 years ago, and no one else has picked up this word. It's a wonder anyone ever saw it. Imagine scribbling your opinion in the margin of a magazine, adding an ending to an existing word, and having your passing fancy — your fancyette — preserved in the eminent dictionary. Just you, that one night, reading Fichte or whatever.

It's strange. How does that get to be a word? It's a wordette. See? I can make a word with a noun and an "-ette." It's easy to do. We do language tricks like that all the time. But how does it get into the OED? Is it a little joke? A jokette? ("Jokette" is not in the OED. "Wordette" is not in the OED.) Or is it something big? — massive reverence for Coleridge.

FROM THE EMAIL: A reader named Iain writes:

Apparently Coleridge was famous for his marginalia. I didn't know this until a few years ago when I read H. J. Jackson's Marginalia: Readers Writing in Books. The author, a professor at University of Toronto, was the editor of the Samuel Taylor Coleridge Marginalia Project, volumes 3-6! (who knew?). I bought the book as soon as I heard of it, and enjoyed it. It came to my attention during a party at the Yale Divinity School, where my wife was pursuing her M.Div. She introduced me to the Divinity School Librarian, and a portion of our conversation went roughly as follows: 

Me: I don't usually take books out of libraries because I write all over my books, and libraries tend to frown on that 

Librarian: Have you ever read Marginalia? 

Me: No. 

Librarian: It's a wonderful exploration of marginalia and its use over time 

Me: Sounds interesting. 

My wife: Oh, that will look great on your bookshelf right next to your history of the footnote 

Librarian: Is that Grafton's history of the footnote? 

Me: Yes it is! 

My wife: I'll leave you two alone now...'bye!

Footnote: Marginalia was the original name of this blog, and the first post on this blog was, in part, about librarians disapproving of marginalia:

This blog is called Marginalia, because I'm writing from Madison, Wisconsin, and Marginalia is a fictionalized name for Madison that I thought up a long time ago when I seriously believed I would write a fictionalized account of my life in Madison, Wisconsin. There is nothing terribly marginal about Madison, really, but I do like writing in the margins of books, something I once caused a librarian to gasp by saying. Writing in a blog is both less and more permanent than writing in the margin of a book.