Mammoth Uncensored Word List:
11 words that are 20 letters long beginning with q in the mammoth uncensored word list.

quadragintilliardths quadringentilliardth quadringentillionths quattuordecilliardth quattuordecillionths quattuorvigintillion quinquagintilliardth quinquagintillionths quinquatrigintillion quinquavigintilliard quinquavigintillions