The longest real word is...
28 letters - Antidisestablishmentarianism
This is the longest word that is accepted without question by everyone. Although some people like to stretch it further with suffixes and prefixes like pseudoantidisestablishmentarianistically. But seriously, no one has ever needed a word for "in the manner of false opposition to the separation of church and state".
The longest "real" word is...
29 letters - Floccinaucinihilipilification
This is the longest word that is in the Oxford English Dictionary. It was coined rather than evolved, hence my quotes, but it has been around long enough that its origins aren't very clear and is widely used by literary geeks everywhere. So it's an honorary real word.
The word everyone thinks is the longest...
34 letters - Supercalifragilisticexpialidocious
Even though this is commonly regarded as The Longest Word in the English Language, it was coined for the sake of being long. If we're going to consider coined words in published works, then Osseocarnisanguineoviscericartilaginonervomedullary (51 letters) from Thomas Love Peacock's Headlong Hall published in 1815 wins by 17 letters.
The longest place name is...
85 letters - Taumatawhakatangihangakoauauotamateaturipukakapikimaungahoronukupokaiwhenuakitanatahu
A hill in New Zealand with a Maori name that translates to "The summit where Tamatea, the man with the big knees, the climber of mountains, the land-swallower who travelled about, played his nose flute to his loved one".
The medical term everyone thinks is the longest...
45 letters - Pneumonoultramicroscopicsilicovolcanoconiosis
This is a lung disease that miners can get that is sometimes shortened to "white lung". Yes, it's a real word, but if we're going to start accepting medical terms into the list of longest English words, then we must also allow this next one in...
An even longer scientific term that arguably is still not the longest...
1913 letters - Methionylglutami nylarginyltyrosy lglutamylserylle ucylphenylalanyl alanylglutaminyl leucyllysylgluta mylarginyllysylg lutamylglycylala nylphenylalanylvalylprolylphenyl alanylvalylthreo nylleucylglycyla spartylprolylgly cylisoleucylglut amylglutaminylse rylleucyllysylis oleucylaspartylt hreonylleucyliso leucylglutamylalanylglycylalanyl aspartylalanylle ucylglutamylleuc ylglycylisoleucy lprolylphenylala nylserylaspartyl prolylleucylalan ylaspartylglycyl prolylthreonylis oleucylglutaminylasparaginylalan ylthreonylleucyl arginylalanylphe nylalanylalanyla lanylglycylvalyl threonylprolylal anylglutaminylcy steinylphenylala nylglutamylmethi onylleucylalanylleucylisoleucyla rginylglutaminyl lysylhistidylpro lylthreonylisole ucylprolylisoleu cylglycylleucyll eucylmethionylty rosylalanylaspar aginylleucylvaly lphenylalanylasparaginyllysylgly cylisoleucylaspa rtylglutamylphen ylalanyltyrosyla lanylglutaminylc ysteinylglutamyl lysylvalylglycyl valylaspartylser ylvalylleucylval ylalanylaspartylvalylprolylvalyl glutaminylglutam ylserylalanylpro lylphenylalanyla rginylglutaminyl alanylalanylleuc ylarginylhistidy lasparaginylvaly lalanylprolyliso leucylphenylalanylisoleucylcyste inylprolylprolyl aspartylalanylas partylaspartylas partylleucylleuc ylarginylglutami nylisoleucylalan ylseryltyrosylgl ycylarginylglycy ltyrosylthreonyltyrosylleucylleu cylserylarginyla lanylglycylvalyl threonylglycylal anylglutamylaspa raginylarginylal anylalanylleucyl prolylleucylaspa raginylhistidyll eucylvalylalanyllysylleucyllysyl glutamyltyrosyla sparaginylalanyl alanylprolylprol ylleucylglutamin ylglycylphenylal anylglycylisoleu cylserylalanylpr olylaspartylglut aminylvalyllysylalanylalanylisol eucylaspartylala nylglycylalanyla lanylglycylalany lisoleucylserylg lycylserylalanyl isoleucylvalylly sylisoleucylisol eucylglutamylglu taminylhistidylasparaginylisoleu cylglutamylproly lglutamyllysylme thionylleucylala nylalanylleucyll ysylvalylphenyla lanylvalylglutam inylprolylmethio nyllysylalanylal anylthreonylarginylserine
This is a really large protein with the formula C1289H2051N343O375S8 and yet, there are arguably even larger scientific words that can created by slapping hundreds of prefixes and suffixes upon other science terms.