Mammoth Uncensored Word List:
11 - 20 letter words starting with q

quadragintilliardths quadringentilliardth quadringentillionths quattuordecilliardth quattuordecillionths quattuorvigintillion quinquagintilliardth quinquagintillionths quinquatrigintillion quinquavigintilliard quinquavigintillions