Mammoth Uncensored Word List:
7 words that are 29 letters long in the mammoth uncensored word list.

cyclotrimethylenetrinitramine floccinaucinihilipilification hexakosioihexekontahexaphobes hexakosioihexekontahexaphobia hexakosioihexekontahexaphobic quinquagintaquadringentillion quinquagintatrecentilliardths