If you allow chemical names -- which are English words -- there is the 84 letter name for the dye "Congo red". I may try to scan it in tomorrow, but I certainly can't type it without making a mistake. A more conventional word is the name for the tenth cranial nerve which is 64 letters in length -- but for obvious reasons commonly just called the vagus nerve. There is a Swedish word that among word gamers is reputed to be the longest word in any language -- over 100 letters as I recall, but I will have to search for it. The 64 letter name of the tenth cranial nerve and the 84 letter chemical name for Congo red I have in hand but am unwilling to try to type.
A common -- that is to say it gets used in conversation occasionally -- 23 letter word is ANTHROPOMORPHOLOGICALLY. I have never heard anyone use it, but Websters has the 25 letter PHILOSOPHICOPSYCHOLOGICAL which I believe is the record in that dictionary.
Added
The vagus nerve's proper name is:
AURICULOLARYNGOPHARYNGOESOPHAGOTRACHEOPULMONOCARDIOGASTROHEPATIC
which is easier to pronounce if hyphenated at each functional break,
AURICULO-LARYNGO-PHARYNGO-ESOPHAGO-TRACHEO-PULMONO-CARDIO-GASTRO-HEPATIC