Titin is the longest 'word' in the English language, being 189, 819 letters long. But as its 'a chemical formulae' some consider it not a word but a collection of words strung together.
It can't be found in dictionary, but when I googled the longest English word that is what I found.

This is video of someone saying the word (I got tired of it after 2 minutes crazy it takes over 8 minutes to say.)

you tube