all of the answers above are wrong,
The longest word in English depends upon the definition of what constitutes an English word. English allows new words to be formed by construction; long words are coined; place names may be considered words; technical terms may be arbitrarily long. Length can be in terms of orthography and number of written letters or phonology and the number of phonemes.
1. Methionylthreonylthreonyl...isoleucine
189,819 letters
Longest full chemical name of a protein
Technical; not in a major dictionary
refer to:
http://en.wikipedia.org/wiki/Methionylthreonylthreonyl...isoleucine
2. Lopado...pterygon
183 letters
Longest word coined by a major author
Coined; not in a major dictionary
refer to:
http://en.wikipedia.org/wiki/Lopado...pterygon
3.Taumatawhakatangihangakoauauotamateapokaiwhenuakitanatahu
85 letters
Longest officially recognized place name
Place name; not in a major dictionary
4. Pneumonoultramicroscopicsilicovolcanoconiosis
45 letters
Longest word in a major dictionary
Technical; coined to be the longest word
5. Pseudopseudohypoparathyroidism
30 letters
Longest non-coined word in a major dictionary
Technical
6. Floccinaucinihilipilification
29 letters
Longest nontechnical word