I was working on my Speech to Text engine, and I needed a cutoff for the word size. I arbitrarily chose 30, which turned out to be the length of ' pseudopseudohypoparathyroidism', which appears in the OED.
On the other hand, the Wikipedia entry for Longest word in English has the following section at the end:
According to an old riddle, smiles may be considered the longest word in English, as there is a mile between the two s's. A retort asserts that beleaguered is longer still, since it contains a league. The riddle and both jocular answers date from the 19th century when standards for humor were considerably lower.
I didn't agree about the standards of humor being high now (considering I still find these funny!), but the last observation apparently references Wayside Gleanings for Leisure Moments, a book published in 1888. There was a second note for that sentence, which says:
Even "longer" words exist (e.g., gigaparsecs, with a gigaparsec before the final s), according to the logic implicit in the jokes.