Mammoth Uncensored Word List:
8 words ending with s, 26 letters long

octillionduotrigintillions quinquagintacentilliardths quinquagintaducentilliards quinquagintatrecentillions quinquaquadragintillionths quinquaquinquagintilliards quinquaseptuagintillionths undecillionsedecilliardths