The longest word in the English dictionary is pneumonoultramicroscopicsilicovolcanoconiosis, which Wikipedia helpfully provides as
a 45-letter word which refers to a lung disease contracted from the inhalation of very fine silica particles, specifically from a volcano. Research has discovered that this word was originally a hoax. It has since been used in a close approximation of its originally intended meaning, lending at least some degree of validity to its claimHow wonderful is that? You can make up a word, and it might still count. There's hope for us plebs yet.
The longest recognised place name, Taumatawhakatangihangakoauauotamateaturipukakapikimaungahoronukupokaiwhenuakitanatahu(88 letters - Wiki Entry here)And of course you can't talk about really long words without mentioning our old favourite, Supercalifragilisticexpialidocious (34 letters- Wiki), but its not a real word and is only included in dictionaries as a Proper Noun, the name of a song.
(Funnily, Blogger's built in Dictionary recognises Super..... but not the two official words above...)