This is rather a meaningless question, since many words are extendable in practice and in theory. Having said that, there are various kinds of �longest' word in English.
Firstly, there are technical words in science and medicine that are just strings of jargon elements. The longest word in The Oxford English Dictionary (TOED) is such a word. It is pneumonoultramicroscopicsilicovolcanoconiosis, (45 letters) which is a lung disease brought on by inhaling certain dust particles. (Mentioned by Atoach above.) The longest of these technical monsters apparently opens with the letters �methionyl...' and goes on for a further 1900 letters!
Secondly, there are joke words that have been made up at various times throughout history, specifically striving for great length. The oldest of these - created at Eton in the 18th century - is probably floccinaucinihilipilification, (29 letters) listed in TOED, which means setting things at little value. A more modern example of such a joke word is supercalifragilisticexpialodocious (34 letters) - also listed in TOED - from the film, 'Mary Poppins'.
Thirdly, there are genuinely long words which have emerged to fill a particular gap in meaning. The longest of those is antidisestablishmentarianism, (28 letters). This might be considered the longest �normal' word in English, in the sense that it is - of the four so far listed - the only one you are at all likely actually to see in print....(cont)