The biggest (i.e., longest) word that appears in the Merriam-Webster dictionaries and the Oxford English Dictionary is the 45-letter PNEUMONOULTRAMICROSPICSILICOVOCANOCONIOSIS. It now appears that this word occurs to to a hoax perpetrated by members of the National Puzzlers League. If true, this is surely one of the greatest ironies on the history of logology.
"The Biggest Hoax,"
Word Ways: Vol. 22
, Article 3.
Available at: http://digitalcommons.butler.edu/wordways/vol22/iss4/3