Mammoth Uncensored Word List:
16 - 20 letter words starting with u

ultracentrifugations ultramicroscopically ultrarevolutionaries ultrasegregationists ultratraditionalists uncharacteristically uncompromisingnesses unconscionablenesses underrepresentations unintelligiblenesses unquinquagintilliard unquinquagintillions unrepresentativeness unsatisfactorinesses unviginticentillions ureterosigmoidostomy