Antidisestablishmentarianism, with 28 letters, is defined in The Oxford English Dictionary (a 20-volume historical dictionary of English) as follows: "Properly, opposition to the disestablishment of the Church of England (rare): but popularly cited as an example of a long word." Merriam-Webster doesn't enter antidisestablishmentarianism in any of its dictionaries because the evidence indicates that the word is almost never used anymore.
The longest word that you'll find in our Webster's Third New International Dictionary, Unabridged, is pneumonoultramicro-scopicsilicovolcanoconiosis, which has 45 letters and which is a disease of the lungs caused by inhaling very fine irritant particles. Miners are especially subject to this disease. However, most use of the word is simply in reference to how long it is. Another long word (27 letters) that you'll find in the Merriam-Webster's Online Dictionary, is electroencephalographically.