Mammoth Uncensored Word List:
19 words that are 20 letters long beginning with s in the mammoth uncensored word list.

salpingooophorectomy semiautobiographical septemvigintillionth septentrigintillions septuagintilliardths sesquinquagintillion sesquipedaliophobics sexagintacentillions sigmoidorectostomies spectrofluorometries spectroheliographies spectrophotometrical sphingophospholipids stereophotogrammetry superrespectableness superresponsibleness superspecializations supersynchronousness syncategorematically