some guy said a B cup which my now deleted post showed that B cups are tiny, and the woman in the pool pic has bigger then B cup, but Billie has bighger tits then the girl in that pool pic
(unless she wears multiple bras and push-ups etc, but the tit slap video where her tits are in either a unsupportive bra or she just isn't wearing a bra her tits look massive, and you cant say its cause of her bra)
but a Quora thing lists her tits as 34 inches (which if you google 34 inch breasts comes up with 34C, but you can get a 34 A,B,D,DD&etc as well so i dunno)
and Strikingly notes her measurements as 'mind your own business' basically
the post i mentioned that got removed included a picture showing different cups
(now obviously cup sizes are done differently around the world)