Mammoth Uncensored Word List:
12 words that are 21 letters long beginning with s in the mammoth uncensored word list.

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