Generally speaking, the longest accepted word is

antidisestablishmentarianism

consisting of 28 letters.

Here is a 45 letter word that refers to the miner’s lung disease silicosis.

pneumonoultramicroscopicsilicovolcanoconiosis

Even though this appears in the Oxford English Dictionary, they do mention that it is “a supposed lung disease“.

This next word contains 182 letters:

Lopadotemachoselachogaleokranioleipsanodrimhypotrimmatosilphioparaomelitokatakechymenokichlepikossyphophattoperisteralektryonoptekephalliokigklopeleiolagoiosiraiobaphetraganopterygon

It was used by Aristophanes (446 BC – 386 BC) in his comedy “Assemblywomen” and it is defined as a fricassee.

Here is a word that is 1,909 letters long:

It is the chemical name for tryptophan synthetase and has the chemical formula:

C_1,289 H_2,051 N₃₄₃ O₃₇₅ S₈

This last word is 189,819 letters long and is the chemical name for the protein “titn”.

And it has the chemical formula:

C_169,719 H_270,466 N_45,688 O_52,238 S₉₁₁Since this is a technical, scientific word which very rarely (if ever) appears in print, it is highly disputed if this can be considered to be an actual word.
Nonetheless, some people consider this to be legitimate.
It is seldom seen printed out in full, but since you are here, these are all of the 189,819 letters of that word: