This is a simple question.
The answer is not so simple. I made a short research on the Internet and here is what I found.
Let’s start with a little joke:
The ‘SMILES’ -is the really longest English word.
Really, there is a MILE between two Ss: SMILES!
The longest English word that can be using only vowels:
(6) Euouae
The longest English word which has all letters in the alphabetical order:
(8) Aegilops
The longest English word that can be constructed with only one vowel:
(9) strengths
The longest English word which you can type only with the left hand:
(10) stewardess
The longest English word which is typable by alternating left and right hands:
(13) dismantlement
The longest English word with no repeated letters:
(15) dermatoglyphics
The longest dune name in English:
(23) Nunathloogagamiutbingoi
The longest English word in Shakespeares’s works:
(28) Honorificabilitudinitatibus
The longest hypothetically legal Scrabble English word:
(28) ethylenediaminetetraacetates
The longest unchallenged nontechnical English word:
(29) Floccinaucinihilipilification
The longest non-coined and nontechnical English word:
(28) Floccinaucinihilipilification
The longest English word in a major dictionary:
(45) Pneumonoultramicroscopicsilicovolcanoconiosis
The longest English lake name:
(45) Chargoggagoggmanchauggagoggchaubunagungamaugg
The longest English hill name:
(85) Taumatawhakatangihangakoauauotamateaturipukakapikimaungahoronukupokaiwhenuakitanatahu
The longest English place name:
(163) Krungthepmahanakornamornratanakosinmahintarayutthayamahadilokphopnopparatrajathaniburiromudomrajaniwesmahasatharnamornphimarnavatarnsathitsakkattiyavisanukamprasit
The longest English word coined by a major author:
(183) Lopadotemachoselachogaleokranioleipsanodrimhypotrimmatosilphioparaomelitokatakechymenokichlepikossyphophattoperisteralektryonoptekephalliokigklopeleiolagoiosiraiobaphetraganopterygon
The longest English Chemical name:
(189,819) Methionylthreonylthreonylglutaminylala…
ylglutaminylprolylleucylglutaminylsery…
lserylthreonylalanylthreonylphenylalan…
rylglycylphenylalanylprolylvalylprolyl…
lanylarginylaspartylglycylglutaminylva…
nylleucylprolylglycylvalylglutaminylis…
ylglycylarginylalanyllysylleucylthreon…
llysylalanylasparaginylserylglycylargi…
…
Note: The longest word cited by the Guinness Book of Records, which allegedly represents the name for human mitochondrial DNA, is more than 207,000 letters long. Some types of DNA could have as many as a billion letters if fully written out.