Take "antidisestablishmentarianism". This is an abstract noun ending in "-ism", so you can turn it into a person noun by changing "-ism" to "-ist": antidisestablishmentarian-ist We add the hyphen just to show what's going on - not because we think it should be spelt that way. Adding "-ic" changes this easily into an adjective: antidisestablishmentarian-ist-ic which "-ally" converts to an adverb: antidisestablishmentarian-ist-ic-ally The Nerdparadise site puts this in the top five, with 34 letters. But wait: who said that expansions could only happen at the end? What about adding a prefix or two?
pre-antidisestablishmentarian-ist-ic-ally mega-pre-antidisestablishmentarian-ist-ic-ally How about that? 41 letters - still 11 short of Number One, but we could go on for ever. The adverb suffix is a dead end (literally) but averbal "-ate" opens up more possibilities and we hit the Guinness Book of Records.
The point is that English gives us tools for building new words out of old material. If you follow the rules, you're bound to get a legal English word, so all these monsters must be English words. (Actually we think Julie Andrews cheated, but that's a matter of opinion.)
As always in English, the language is a resource - a DIY vocabulary-construction kit. But as in all DIY, your wonderful precision work may turn out to be completely useless. Who wants a shelf for holding books three inches tall? And who wants a word for talking about the attitude of one who opposes the end of the established Church of England? Looking for "the longest word" may be fun for students, but it leads neatly into word-building, which is an important part of any writer's apprenticeship because it helps them to understand unfamiliar words and to invent new words of their own. Take the longest word they can think of, then decompose it into its parts, then show how those parts crop up in other words and how they can recycle them for their own use. The starting-point needn't be very long. For example, take "concentration". Show how this is made up: con-centr-at-ion Link it to other words like "centre", "con-tain", "don-ate", "illustr-at-ion" and (of course) "con-centr-ate"; discuss the meanings and forms; then ask them to produce a noun for "frustrate". And so on. After a few sessions like this they may be ready to challenge the world record. On the other hand this may be a waste of energisationalism.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk