But what is th wrong one? First of, one that is clarly the wrong size. Okay, guys won't notice much of that, but you will. And if it's as uncomfortable as having the wrong boxer size, then i'd really suggest getting the right size. Secondly, it depends on social setting and clothes. You don't wear a bra with straps if you are to use a dress without anything over your shoulders, for example. Nor do you pick the oldest one you got if you plan to seduce your boyfriend. Or if you're doing sports, you'd rather want one that support well.
So the right size would be one that fits well on you, and that fits the social context and your clothes. "but guys only care abotu big tits". Classic claim, but the truth is we don't. BOYS only care about that, real men don't. They're awesome wether they're melons or can fit in a wine glass. Yes, we got preferences, but at the end of the day they're just different. They're all good, so just be comfortable and proud of the ones you got. That's what REALLY matters.
Don't think of a bikini in terms of size, think in terms of comfort. How comfortable are you in your bikini? Are you making others uncomfortable by what you're wearing? (I only say that because not at every event are certain bathing suits appropriate, not around certain crowds, etc.)