The longest 'word' you might find (unless someone strives to make a longer one) is the chemical name for titin, the largest known protein. It contains 189,819 letters. It is, of course, a technical term, not found in any dictionary.
The longest word you could find in an English dictionary (although few dictionaries contain it) is Pneumonoultramicroscopicsilicovolcanocon... as mentioned above. It contains 45 letters and is a coined technical term.
The longest non-coined technical word is Pseudopseudohypoparathyroidism, containing 30 letters.
The longest coined, nontechnical word is Floccinaucinihilipilification, containing 29 letters.
The longest non-coined, nontechnical word in English is Antidisestablishmentarianism, at 28 letters.
The choice for what is the longest word in English depends on what the criteria are for something to be a 'word' in English.