“Methionylthreonylthreonylglutaminylarginyl…isoleucine”, the chemical name of “titin” (also known as “connectin”) is the largest known protein and is considered being the longest word in English. It has 189,819 letters.
According to the Oxford English Dictionary, the longest word is “pneumonoultramicroscopicsilicovolcanokoniosis” at 45 letters long. It is a lung disease caused by the inhalation of silica or quartz dust.
Among other longest words, one can find a 34 letters long “supercalifragilisticexpialidocious”, made famous by Mary Poppins.
The 21 letter word “incomprehensibilities” is known as the longest used in common language.