Mammoth Uncensored Word List:
14 words that are 18 letters long beginning with q in the mammoth uncensored word list.

quadragintilliards quadragintillionth quadringentilliard quadringentillions quantificationally quantitativenesses quasiperiodicities quattuordecilliard quattuordecillions questionablenesses quingentilliardths quinquadecillionth quinquagintilliard quinquagintillions