Not to make this overly complicated, but I can't go by size alone. For one, I'm more turned on by the size and shape of a woman's nipples and areolas than her boobs. For another, by just saying a size, I'd have to include bad boob jobs just because they meet some meaningless cup size requirement.
To me, Ice La Fox (pre bad boob job, with A-B cups) was every bit as sexy as Brandy Talore (with natural DD-DDD cups)... because they both had fantastic nipples. Now, Ice is probably a D cup and she (IMO) looks hideous. Same goes for Lucy Lee (Asian). Girls with small boobs, but great nipples who now have big silly-cones. /thumbsdown/
So I like whatever size (natural or well done make believe) boobs that are topped with large erect nipples or puffies.