Mammoth Uncensored Word List:
18 words ending with ss, 19 letters long

contemporaneousness disadvantageousness distinguishableness interchangeableness nonreproductiveness nonsubconsciousness overimaginativeness overpresumptiveness overspeculativeness oversusceptibleness straightforwardness unaccommodatingness unapprehendableness unauthoritativeness uncommunicativeness unconscientiousness undemonstrativeness unexceptionableness