Mammoth Uncensored Word List:
19 - 20 letter words starting with s

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