For me the ideal size is a G-cup, and I'm betting this will become a very standard size (in porn industry) in a few years.
We already have some big name with a G-cup :
natural / soft :
Bea Flora
Busty Ellen
Faith Nelson
Hannah Minx
Jana Defi
Merilyn Sakova
Michelle Marsh
Monica Mendez
Peta Todd
Satinee Capona
Shawna Elizabeth
Stacey Poole
Wendy Star
Yulia Nova
fake / soft :
Bella French
Brandy Robbins
Bryci
Desiree-Elyda Villalobos
Jenny Poussin
Lindsay Walker
Marika Fruscio
natural / hard :
Alia Janine
Arianna Sinn
Cassandra
Tyra Moore
fake / hard :
Breanna Sparks
Candy Manson
Chloe Reece Ryder
Daphne Rosen
Scarlet Lovatt
Delta White
Eve Deluxe