This is a debatable question.
The original French name for smurf is schtroumpf, and is used as an all-purpose noun and verb by Smurfs and in imitation of them. The form schtroumpfed is used in Alistair McEwen's English translation of an essay by Umberto Eco: "Let us suppose that an English speaker of average culture hears a Schtroumpf poet reciting I schtroumpfed lonely as a schtroumpf." However, this is intended to represent the Schtroumpf language rather than English. That one syllable word is twelve letters.
The eleven-letter word broughammed (created from brougham by analogy with bussed, biked, carted etc.), while readily pronounceable as one syllable in all dialects ("broomed", IPA: /bɹu¢°md/), is yet to appear in a print dictionary. See: "ough" words. The word might also be spelled broughamed, with ten letters.
Squirrelled is the spelling in British English of a word usually spelled in American English as squirreled (see -led and -lled spellings) . While in Received Pronunciation the word has two syllables (IPA: /ˈskwɪɹəɫd/), it is often pronounced /skwɝɫd/ (rhymes with world) in North American English [1]. Of those who use the one-syllable pronunciation, some may use the eleven-letter spelling; for the rest, it is a ten-letter monosyllable.
https://en.wikipedia.org/wiki/List_of_the_longest_English_words_with_one_syllable