Mammoth Uncensored Word List:
19 - 16 letter words starting with w

waterdispersible waterproofnesses watertightnesses weatherabilities weatherboardings weatherproofness weatherresistant weatherstrippers weatherstripping weathertightness weightlessnesses weltanschauungen whatchamacallits wholeheartedness willingheartedly wondermongerings worshipfulnesses worthwhilenesses wrongheartedness