Originally posted by Andrew Kern here is a 10 letter one: squirreled
I went and looked. (10 or 11 letters) but you are right, and I'm surprised that the way I pronounce it agrees with 3% of the UK english speakers. Wiki says A list of 9,123 English monosyllables published in 1957 includes three ten-letter [lol] words: scraunched, scroonched, and squirrelled. It also says that squirrelled is compressed American pronunciation of a word which in British RP always has two syllables /ˈskwɪrəld/. The monosyllabic pronunciation rhymes with world, curled.
Also, Guinness World Records lists scraunched and strengthed. So I was pretty close! I wonder what scraunch and scroonch mean.