Mammoth Uncensored Word List:
23 - 14 letter words starting with be

bearablenesses beatifications beautification becomingnesses beggarlinesses behaviouristic beleaguerments belletristical belligerencies benefactresses beneficialness beneficiations benevolentness benumbednesses benzanthracene benzimidazoles benzodiazepine benzoxycamphor berberidaceous beseechingness besottednesses bestsellerdoms bewilderedness