Let us consider only words published by many dictionaries in English. There are many other words which are coined and used in one or two places. However, they are disputed - whether they are accepted words or not.
1. antidisestablishmentarianism .
it has 27 letters
It has 45 letters - it means a disease in the lungs.
3. Floccinaucinihilipilification - means the habit of
estimating as worthless. This is not in many dictionaries. It is present in one dictionary. It has 29 letters.
- means " with honor "
it is in dictionary . it has 27 letters
5. Methionylthreonylthreonylglutaminylarginyl...isoleucine - a protein name. 189,919 letters - it takes 3 hours to complete pronunciation
6. Methionylglutaminylarginyltyrosylglutamyl...serine - protein name - 1909 letters long.
7. Lopadotemachoselachogaleokranioleipsano...pterygon - translation from Greek perhaps. - 175 letters
there are other words - quite long, but not offiicial and used by many. they were invented for fun or for record.