Mammoth Uncensored Word List:
12 - 21 letter words starting with s

salpingoophorectomies septemvigintillionths septentrigintillionth septuagintacentillion sesquinquagintillions sexagintacentillionth sigmoidoproctostomies sphygmomanometrically spondyloarthropathies stereomicroscopically straightforwardnesses syncytiotrophoblastic