well, according to The Old Farmer's Almanac (I'm just Old, so can't vouch for accuracy) The longest word you can spell without repeating a letter: uncopyrightable

here's some more verbal trumpery from the same source, which we can use perhaps to judge the veracity of same...
The longest word with just one vowel: strengths
The only English word with a triple letter: goddessship
The word with the longest definition in most dictionaries: set
The shortest "ology" (study of) word: oology (the study of birds' eggs)
The only word in which an "f" is pronounced like a "v": of