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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

2.  pneumonoultramicro-scopicsilicovolcanoconiosis
      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.   

4.  Honorificabilitudini-tatibus
        -  means  " with honor   "
             it is in   dictionary .  it has 27 letters

Methionylthreonylthreonylglutaminylarginyl...isoleucine  -  a protein name. 189,919 letters   - it takes 3 hours to complete pronunciation   

Methionylglutaminylarginyltyrosylglutamyl...serine    -   protein name  - 1909 letters long.   

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.

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You should not give wrong answers