Mammoth Uncensored Word List:
16 - 14 letter words starting with du

ducentilliards ducentillionth dulcifications dumbfoundering dumbfoundingly dunderheadisms duocentillions duodecilliards duodecillionth duodenectomies duodenojejunal duodenostomies duononagesimal duosexagesimal duotrigesimals durchkomponirt